code: func (h *HTTPBin) ResponseHeaders(w http.ResponseWriter, r *http.Request) {
  	args := r.URL.Query()
  	for k, vs := range args {
  		for _, v := range vs {
  			w.Header().Add(http.CanonicalHeaderKey(k), v)
  		}
  	}
  	body, _ := json.Marshal(args)
  	if contentType := w.Header().Get("Content-Type"); contentType == "" {
  		w.Header().Set("Content-Type", jsonContentType)
  	}
  	w.Write(body)
  }
docs: // ResponseHeaders responds with a map of header values