mux.go
func (mx *Mux) NotFoundHandler() http.HandlerFunc {
	if mx.notFoundHandler != nil {
		return mx.notFoundHandler
	}
	return http.NotFound
}
func (mx *Mux) NotFoundHandler() http.HandlerFunc {
	if mx.notFoundHandler != nil {
		return mx.notFoundHandler
	}
	return http.NotFound
}
// NotFoundHandler returns the default Mux 404 responder whenever a route
// cannot be found.