code: func (m *mux) HandleMethodNotAllowed(handle MethodNotAllowedHandler) {
  	mna := func(rw http.ResponseWriter, req *http.Request, methods map[string]httptreemux.HandlerFunc) {
  		handle(rw, req, nil, methods)
  	}
  	m.router.MethodNotAllowedHandler = mna
  }
docs: // HandleMethodNotAllowed sets the MuxHandler invoked for requests that match
  // the path of a handler but not its HTTP method.