code: func (w *ResponseWriter) Header() http.Header {
  	if w.headers == nil {
  		w.headers = make(http.Header)
  	}
  	return w.headers
  }
docs: // Header returns the header map that will be sent by
  // WriteHeader. Changing the header after a call to
  // WriteHeader (or Write) has no effect unless the modified
  // headers were declared as trailers by setting the
  // "Trailer" header before the call to WriteHeader (see example).
  // To suppress implicit response headers, set their value to nil.