[BUG] go-server doesn't honour the provided headers within EncodeJSONResponse

Created by: mgoltzsche

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

When the option addResponseHeaders is enabled for the go-server generator it generates a ResponseWithHeaders method that calls an EncodeJSONResponse method which however ignores the headers that are provided to it. The generated method looks as follows:

func EncodeJSONResponse(i interface{}, status *int, headers map[string][]string, w http.ResponseWriter) error {
	w.Header().Set("Content-Type", "application/json; charset=UTF-8")
	if status != nil {
		w.WriteHeader(*status)
	} else {
		w.WriteHeader(http.StatusOK)
	}

	return json.NewEncoder(w).Encode(i)
}

This stops me from specifying response headers within my controller implementation.

I would expect this method to apply the headers I provided.

openapi-generator version

openapi-generator 5.1.1

OpenAPI declaration file content or url

This is the example OpenAPI schema example-openapi.yaml - though you could use any other to reproduce the bug that is described here:

openapi: "3.0.3"

info:
  title: "Example API"
  version: "1.0.0"

paths:
  "/path/{myparam}/myresource":
    post:
      operationId: createResource
      parameters:
      - name: myparam
        in: path
        required: true
        schema:
          type: string
      requestBody:
        content:
          "application/json":
            schema:
              type: object
              properties:
                name:
                  type: string
      responses:
        '201':
          description: response with headers
          headers:
            myheader:
              schema:
                type: string

Generation Details

java -jar ./openapi-generator-5.1.1.jar generate -i ./example-openapi.yaml -g go-server -o ./generated --api-name-suffix API --package-name generated -p addResponseHeaders=true

Steps to reproduce

see generation details

Related issues/PRs

Suggest a fix

Generate the EncodeJSONResponse method so that it applies the provided headers to the ResponseWriter:

func EncodeJSONResponse(i interface{}, status *int, headers map[string][]string, w http.ResponseWriter) error {
	w.Header().Set("Content-Type", "application/json; charset=UTF-8")
	if headers != nil {
		for k, v := range headers {
			w.Header()[k] = v
		}
	}
	if status != nil {
		w.WriteHeader(*status)
	} else {
		w.WriteHeader(http.StatusOK)
	}

	return json.NewEncoder(w).Encode(i)
}