code: func (r *Response) WriteHeader(code int) {
  	if r.committed {
  		log.Println("response already committed")
  		return
  	}
  	r.status = code
  	r.ResponseWriter.WriteHeader(code)
  	r.committed = true
  }
docs: // 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.