mux.go
func (mx *Mux) HandleFunc(pattern string, handlerFn http.HandlerFunc) {
	mx.handle(mALL, pattern, handlerFn)
}
func (mx *Mux) HandleFunc(pattern string, handlerFn http.HandlerFunc) {
	mx.handle(mALL, pattern, handlerFn)
}
// HandleFunc adds the route `pattern` that matches any http method to
// execute the `handlerFn` http.HandlerFunc.