splithttp: Add support for H2C and http/1.1 ALPN on server (#3465)

* Add H2C support to server

* update comment

* Make http1.1 ALPN work on SplitHTTP client

Users that encounter protocol version issues will likely try to set the
ALPN explicitly. In that case we should simply grant their wish, because
the intent is obvious.

transport/internet/splithttp/dialer.go

@@ -58,6 +58,7 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 	}
 
 	tlsConfig := tls.ConfigFromStreamSettings(streamSettings)
+	isH2 := tlsConfig != nil && !(len(tlsConfig.NextProtocol) == 1 && tlsConfig.NextProtocol[0] == "http/1.1")
 
 	var gotlsConfig *gotls.Config
 
@@ -88,7 +89,7 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 	var uploadTransport http.RoundTripper
 	var downloadTransport http.RoundTripper
 
-	if tlsConfig != nil {
+	if isH2 {
 		downloadTransport = &http2.Transport{
 			DialTLSContext: func(ctxInner context.Context, network string, addr string, cfg *gotls.Config) (net.Conn, error) {
 				return dialContext(ctxInner)
@@ -121,7 +122,7 @@ func getHTTPClient(ctx context.Context, dest net.Destination, streamSettings *in
 		upload: &http.Client{
 			Transport: uploadTransport,
 		},
-		isH2:           tlsConfig != nil,
+		isH2:           isH2,
 		uploadRawPool:  &sync.Pool{},
 		dialUploadConn: dialContext,
 	}