code: func NotAllowed(w http.ResponseWriter, r *http.Request) {
  	handlerMu.RLock()
  	f, ok := handlerMap[http.StatusMethodNotAllowed]
  	handlerMu.RUnlock()
  	if ok {
  		f.ServeHTTP(w, r)
  	} else {
  		defaultNotAllowed(w, r)
  	}
  }
docs: // NotAllowed returns a generic HTTP 405 Not Allowed status and response body
  // to the client.