// see https://github.com/golang/go/issues/56352.
// This means we allow only HTTP/1.1 or HTTP/2 prior knowledge.
func NewHandler(h http.Handler, s *http2.Server) http.Handler {
	return denyH2cUpgrade(h2c.NewHandler(h, s))
}

func denyH2cUpgrade(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		if isH2CUpgrade(r.Header) {
			w.WriteHeader(http.StatusMethodNotAllowed)