Pass Content-Type Header to Interceptor

`Content-Type` should be passed to interceptor while making POST Request. Some
framework in different languages like java require this. So interceptor request
fails with 415 in those.

pkg/interceptors/interceptors.go

@@ -32,6 +32,7 @@ import (
 
 const (
 	CoreInterceptorsHost = "tekton-triggers-core-interceptors"
+	ContentType          = "application/json"
 )
 
 // Interceptor is the interface that all interceptors implement.
@@ -124,6 +125,9 @@ func Execute(ctx context.Context, client *http.Client, req *triggersv1beta1.Inte
 	if err != nil {
 		return nil, err
 	}
+
+	r.Header.Set("Content-Type", ContentType)
+
 	res, err := client.Do(r)
 	if err != nil {
 		return nil, err

pkg/interceptors/interceptors_test.go

@@ -354,6 +354,21 @@ func TestResolveToURL(t *testing.T) {
 	})
 }
 
+type localT struct {
+	T *testing.T
+}
+
+func (t localT) testHeader(next http.Handler) http.Handler {
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		contentType := r.Header.Get("Content-Type")
+		if contentType != interceptors.ContentType {
+			t.T.Fatalf("Header expected to be: %s but got :%s",
+				interceptors.ContentType, contentType)
+		}
+		next.ServeHTTP(w, r)
+	})
+}
+
 // testServer creates a httptest server with the passed in handler and returns a http.Client that
 // can be used to talk to these interceptors
 func testServer(t testing.TB, handler http.Handler, d ...*http.Transport) *http.Client {
@@ -427,7 +442,13 @@ func TestExecute(t *testing.T) {
 			if err != nil {
 				t.Fatalf("failed to initialize core interceptors: %v", err)
 			}
-			httpClient := testServer(t, coreInterceptors)
+
+			lT := localT{
+				T: t,
+			}
+
+			handler := lT.testHeader(coreInterceptors)
+			httpClient := testServer(t, handler)
 			got, err := interceptors.Execute(context.Background(), httpClient, tc.req, tc.url)
 			if err != nil {
 				t.Fatalf("Execute() unexpected error: %s", err)
@@ -446,7 +467,7 @@ func TestExecute_Error(t *testing.T) {
 			"Content-Type": {"application/json"},
 		}),
 		Context: &triggersv1.TriggerContext{
-			EventURL:  "http://someurl.com",
+			EventURL:  "http://somurel.com",
 			EventID:   "abcde",
 			TriggerID: "namespaces/default/triggers/test-trigger",
 		},