Google App Engine changes content-type to text/html even though it is set to application/xml

This question has been asked before but that answer applies to python applications. I'd like to know how to fix the issue for go applications.

I've deployed a web service on Google App Engine which is consumed by mobile clients. Using the function below, I send the response either as XML or JSON (as requested by the client)

func (api *API) Respond(w http.ResponseWriter, r *http.Request, body interface{}, status int) {

	var contentType string
	var content []byte
	var err error

	if r.Header.Get("Accept") == "application/xml" {

		contentType = "application/xml; charset=UTF-8"
		content, err = xml.Marshal(body)

	} else {

		contentType = "application/json; charset=UTF-8"
		content, err = json.MarshalIndent(body, "", "  ")
	}

	if err != nil {
		panic(err)
	}

	w.WriteHeader(status)
	w.Header().Set("Content-Type", contentType)
	w.Write(content)
}

In either case, however, the client device receives a Content-Type of text/html. How can I fix this? Here's the app.yam file for my application:

application: wks-api
version: 3
runtime: go
api_version: go1

handlers:
- url: /.*
  script: api

Look at the documentation in https://golang.org/pkg/net/http/#ResponseWriter , and I quote:

> Changing the header after a call to WriteHeader (or Write) has no
> effect

Now look at your code:

w.WriteHeader(status)
w.Header().Set("Content-Type", contentType)

as you see, you are indeed "Changing the header after a call to WriteHeader" -- hence, it "has no effect".

So, do your "changing the header" before that call instead:

w.Header().Set("Content-Type", contentType)
w.WriteHeader(status)

I don't think this is at all specific to app engine -- it should apply to any use of http in Go.

See information from http/header:

> WriteHeader sends an HTTP response header with status code. If
> WriteHeader is not called explicitly, the first call to Write will
> trigger an implicit WriteHeader(http.StatusOK). Thus explicit calls
> to WriteHeader are mainly used to send error codes.

first try settings the header then send it