Allow setting headers in HTTP transports

Currently gqlgen sets Content-Type header to 'application/json'. There's
no easy way to change it or add additional headers.

This commit adds struct variable ResponseHeaders that can hold any
headers you want to be returned with response. It is standard
`map[string][]string` variable.

If user does not set this map, we default to the Content-Type
header with 'application/json' value - nothing will be changed
for existing users.

Usage:

as simple as:

```
headers := map[string][]string{
    "Content-Type": {"application/json; charset: utf8"},
    "Other-Header": {"dummy-post-header","another-value"},
}

h.AddTransport(transport.POST{ResponseHeaders: headers})
```

Added tests in transport/headers_test.go.

graphql/handler/transport/headers.go

@@ -0,0 +1,17 @@
+package transport
+
+import "net/http"
+
+func writeHeaders(w http.ResponseWriter, headers map[string][]string) {
+	if len(headers) == 0 {
+		headers = map[string][]string{
+			"Content-Type": {"application/json"},
+		}
+	}
+
+	for key, values := range headers {
+		for _, value := range values {
+			w.Header().Add(key, value)
+		}
+	}
+}

graphql/handler/transport/headers_test.go

@@ -0,0 +1,163 @@
+package transport_test
+
+import (
+	"context"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+
+	"github.com/99designs/gqlgen/graphql"
+	"github.com/99designs/gqlgen/graphql/handler"
+	"github.com/99designs/gqlgen/graphql/handler/testserver"
+	"github.com/99designs/gqlgen/graphql/handler/transport"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+	"github.com/vektah/gqlparser/v2"
+	"github.com/vektah/gqlparser/v2/ast"
+)
+
+func TestHeadersWithPOST(t *testing.T) {
+	t.Run("Headers not set", func(t *testing.T) {
+		h := testserver.New()
+		h.AddTransport(transport.POST{})
+
+		resp := doRequest(h, "POST", "/graphql", `{"query":"{ name }"}`)
+		assert.Equal(t, http.StatusOK, resp.Code)
+		assert.Equal(t, 1, len(resp.Header()))
+		assert.Equal(t, "application/json", resp.Header().Get("Content-Type"))
+	})
+
+	t.Run("Headers set", func(t *testing.T) {
+		headers := map[string][]string{
+			"Content-Type": {"application/json; charset: utf8"},
+			"Other-Header": {"dummy-post", "another-one"},
+		}
+
+		h := testserver.New()
+		h.AddTransport(transport.POST{ResponseHeaders: headers})
+
+		resp := doRequest(h, "POST", "/graphql", `{"query":"{ name }"}`)
+		assert.Equal(t, http.StatusOK, resp.Code)
+		assert.Equal(t, 2, len(resp.Header()))
+		assert.Equal(t, "application/json; charset: utf8", resp.Header().Get("Content-Type"))
+		assert.Equal(t, "dummy-post", resp.Header().Get("Other-Header"))
+		assert.Equal(t, "another-one", resp.Header().Values("Other-Header")[1])
+	})
+}
+
+func TestHeadersWithGET(t *testing.T) {
+	t.Run("Headers not set", func(t *testing.T) {
+		h := testserver.New()
+		h.AddTransport(transport.GET{})
+
+		resp := doRequest(h, "GET", "/graphql?query={name}", "")
+		assert.Equal(t, http.StatusOK, resp.Code)
+		assert.Equal(t, 1, len(resp.Header()))
+		assert.Equal(t, "application/json", resp.Header().Get("Content-Type"))
+	})
+
+	t.Run("Headers set", func(t *testing.T) {
+		headers := map[string][]string{
+			"Content-Type": {"application/json; charset: utf8"},
+			"Other-Header": {"dummy-get"},
+		}
+
+		h := testserver.New()
+		h.AddTransport(transport.GET{ResponseHeaders: headers})
+
+		resp := doRequest(h, "GET", "/graphql?query={name}", "")
+		assert.Equal(t, http.StatusOK, resp.Code)
+		assert.Equal(t, 2, len(resp.Header()))
+		assert.Equal(t, "application/json; charset: utf8", resp.Header().Get("Content-Type"))
+		assert.Equal(t, "dummy-get", resp.Header().Get("Other-Header"))
+	})
+}
+
+func TestHeadersWithMULTIPART(t *testing.T) {
+	t.Run("Headers not set", func(t *testing.T) {
+		es := &graphql.ExecutableSchemaMock{
+			ExecFunc: func(ctx context.Context) graphql.ResponseHandler {
+				return graphql.OneShot(graphql.ErrorResponse(ctx, "not implemented"))
+			},
+			SchemaFunc: func() *ast.Schema {
+				return gqlparser.MustLoadSchema(&ast.Source{Input: `
+					type Mutation {
+						singleUpload(file: Upload!): String!
+					}
+					scalar Upload
+				`})
+			},
+		}
+
+		h := handler.New(es)
+		h.AddTransport(transport.MultipartForm{})
+
+		es.ExecFunc = func(ctx context.Context) graphql.ResponseHandler {
+			return graphql.OneShot(&graphql.Response{Data: []byte(`{"singleUpload":"test"}`)})
+		}
+
+		operations := `{ "query": "mutation ($file: Upload!) { singleUpload(file: $file) }", "variables": { "file": null } }`
+		mapData := `{ "0": ["variables.file"] }`
+		files := []file{
+			{
+				mapKey:      "0",
+				name:        "a.txt",
+				content:     "test1",
+				contentType: "text/plain",
+			},
+		}
+		req := createUploadRequest(t, operations, mapData, files)
+
+		resp := httptest.NewRecorder()
+		h.ServeHTTP(resp, req)
+		require.Equal(t, http.StatusOK, resp.Code, resp.Body.String())
+		assert.Equal(t, 1, len(resp.Header()))
+		assert.Equal(t, "application/json", resp.Header().Get("Content-Type"))
+	})
+
+	t.Run("Headers set", func(t *testing.T) {
+		es := &graphql.ExecutableSchemaMock{
+			ExecFunc: func(ctx context.Context) graphql.ResponseHandler {
+				return graphql.OneShot(graphql.ErrorResponse(ctx, "not implemented"))
+			},
+			SchemaFunc: func() *ast.Schema {
+				return gqlparser.MustLoadSchema(&ast.Source{Input: `
+					type Mutation {
+						singleUpload(file: Upload!): String!
+					}
+					scalar Upload
+				`})
+			},
+		}
+
+		h := handler.New(es)
+		headers := map[string][]string{
+			"Content-Type": {"application/json; charset: utf8"},
+			"Other-Header": {"dummy-multipart"},
+		}
+		h.AddTransport(transport.MultipartForm{ResponseHeaders: headers})
+
+		es.ExecFunc = func(ctx context.Context) graphql.ResponseHandler {
+			return graphql.OneShot(&graphql.Response{Data: []byte(`{"singleUpload":"test"}`)})
+		}
+
+		operations := `{ "query": "mutation ($file: Upload!) { singleUpload(file: $file) }", "variables": { "file": null } }`
+		mapData := `{ "0": ["variables.file"] }`
+		files := []file{
+			{
+				mapKey:      "0",
+				name:        "a.txt",
+				content:     "test1",
+				contentType: "text/plain",
+			},
+		}
+		req := createUploadRequest(t, operations, mapData, files)
+
+		resp := httptest.NewRecorder()
+		h.ServeHTTP(resp, req)
+		require.Equal(t, http.StatusOK, resp.Code, resp.Body.String())
+		assert.Equal(t, 2, len(resp.Header()))
+		assert.Equal(t, "application/json; charset: utf8", resp.Header().Get("Content-Type"))
+		assert.Equal(t, "dummy-multipart", resp.Header().Get("Other-Header"))
+	})
+}

graphql/handler/transport/http_form.go

@@ -20,6 +20,10 @@ type MultipartForm struct {
 	// as multipart/form-data in memory, with the remainder stored on disk in
 	// temporary files.
 	MaxMemory int64
+
+	// Map of all headers that are added to graphql response. If not
+	// set, only one header: Content-Type: application/json will be set.
+	ResponseHeaders map[string][]string
 }
 
 var _ graphql.Transport = MultipartForm{}
@@ -52,7 +56,7 @@ func (f MultipartForm) maxMemory() int64 {
 }
 
 func (f MultipartForm) Do(w http.ResponseWriter, r *http.Request, exec graphql.GraphExecutor) {
-	w.Header().Set("Content-Type", "application/json")
+	writeHeaders(w, f.ResponseHeaders)
 
 	start := graphql.Now()
 

graphql/handler/transport/http_get.go

@@ -15,7 +15,11 @@ import (
 
 // GET implements the GET side of the default HTTP transport
 // defined in https://github.com/APIs-guru/graphql-over-http#get
-type GET struct{}
+type GET struct {
+	// Map of all headers that are added to graphql response. If not
+	// set, only one header: Content-Type: application/json will be set.
+	ResponseHeaders map[string][]string
+}
 
 var _ graphql.Transport = GET{}
 
@@ -34,7 +38,7 @@ func (h GET) Do(w http.ResponseWriter, r *http.Request, exec graphql.GraphExecut
 		writeJsonError(w, err.Error())
 		return
 	}
-	w.Header().Set("Content-Type", "application/json")
+	writeHeaders(w, h.ResponseHeaders)
 
 	raw := &graphql.RawParams{
 		Query:         query.Get("query"),

graphql/handler/transport/http_post.go

@@ -14,7 +14,11 @@ import (
 
 // POST implements the POST side of the default HTTP transport
 // defined in https://github.com/APIs-guru/graphql-over-http#post
-type POST struct{}
+type POST struct {
+	// Map of all headers that are added to graphql response. If not
+	// set, only one header: Content-Type: application/json will be set.
+	ResponseHeaders map[string][]string
+}
 
 var _ graphql.Transport = POST{}
 
@@ -44,7 +48,7 @@ func getRequestBody(r *http.Request) (string, error) {
 
 func (h POST) Do(w http.ResponseWriter, r *http.Request, exec graphql.GraphExecutor) {
 	ctx := r.Context()
-	w.Header().Set("Content-Type", "application/json")
+	writeHeaders(w, h.ResponseHeaders)
 	params := &graphql.RawParams{}
 	start := graphql.Now()
 	params.Headers = r.Header