Fix marshaling of aliased custom types

This should fix #579 by detecting aliasing of Date and adding
JSON marshal/unmarshal hooks to the alias.

internal/test/issues/issue-312/spec.yaml

@@ -1,4 +1,3 @@
-
 openapi: "3.0.0"
 info:
   version: 1.0.0

internal/test/issues/issue-579/gen.go

@@ -0,0 +1,3 @@
+package issue_579
+
+//go:generate go run github.com/deepmap/oapi-codegen/cmd/oapi-codegen --package=issue_579 --generate=types,skip-prune -o issue.gen.go spec.yaml

internal/test/issues/issue-579/issue_test.go

@@ -0,0 +1,14 @@
+package issue_579
+
+import (
+	"encoding/json"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+)
+
+func TestAliasedDate(t *testing.T) {
+	pet := Pet{}
+	err := json.Unmarshal([]byte(`{"born": "2022-05-19", "born_at": "2022-05-20"}`), &pet)
+	require.NoError(t, err)
+}

internal/test/issues/issue-579/spec.yaml

@@ -0,0 +1,27 @@
+openapi: "3.0.0"
+info:
+  version: 1.0.0
+  title: Issue 579 test
+paths:
+  /placeholder:
+    get:
+      responses:
+        200:
+          description: placeholder
+          content:
+            text/plain:
+              schema:
+                type: string
+components:
+  schemas:
+    Pet:
+      type: object
+      properties:
+        born:
+          $ref: "#/components/schemas/AliasedDate"
+        born_at:
+          type: string
+          format: date
+    AliasedDate:
+      type: string
+      format: date

pkg/codegen/schema.go

@@ -315,7 +315,7 @@ func GenerateGoSchema(sref *openapi3.SchemaRef, path []string) (Schema, error) {
 		}
 		return outSchema, nil
 	} else if len(schema.Enum) > 0 {
-		err := resolveType(schema, path, &outSchema)
+		err := oapiSchemaToGoType(schema, path, &outSchema)
 		if err != nil {
 			return Schema{}, fmt.Errorf("error resolving primitive type: %w", err)
 		}
@@ -352,16 +352,17 @@ func GenerateGoSchema(sref *openapi3.SchemaRef, path []string) (Schema, error) {
 		}
 		//outSchema.RefType = typeName
 	} else {
-		err := resolveType(schema, path, &outSchema)
+		err := oapiSchemaToGoType(schema, path, &outSchema)
 		if err != nil {
 			return Schema{}, fmt.Errorf("error resolving primitive type: %w", err)
 		}
 	}
 	return outSchema, nil
 }
 
-// resolveType resolves primitive  type or array for schema
-func resolveType(schema *openapi3.Schema, path []string, outSchema *Schema) error {
+// oapiSchemaToGoType converts an OpenApi schema into a Go type definition for
+// all non-object types.
+func oapiSchemaToGoType(schema *openapi3.Schema, path []string, outSchema *Schema) error {
 	f := schema.Format
 	t := schema.Type