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