mux.go
func (mx *Mux) MethodNotAllowedHandler() http.HandlerFunc {
	if mx.methodNotAllowedHandler != nil {
		return mx.methodNotAllowedHandler
	}
	return methodNotAllowedHandler
}
func (mx *Mux) MethodNotAllowedHandler() http.HandlerFunc {
	if mx.methodNotAllowedHandler != nil {
		return mx.methodNotAllowedHandler
	}
	return methodNotAllowedHandler
}
// MethodNotAllowedHandler returns the default Mux 405 responder whenever
// a method cannot be resolved for a route.