reverse_proxy: fix bidirectional streams with encodings (fix #3606) (#3620)

* reverse_proxy: fix bi-h2stream breaking gzip encode handle(#3606).

* reverse_proxy: check http version of both sides to avoid affecting non-h2 upstream.

* Minor cleanup; apply review suggestions

Co-authored-by: Matthew Holt <mholt@users.noreply.github.com>

modules/caddyhttp/reverseproxy/reverseproxy.go

@@ -613,8 +613,8 @@ func (h *Handler) reverseProxy(rw http.ResponseWriter, req *http.Request, di Dia
 
 	// some apps need the response headers before starting to stream content with http2,
 	// so it's important to explicitly flush the headers to the client before streaming the data.
-	// (see https://github.com/caddyserver/caddy/issues/3556 for use case)
-	if req.ProtoMajor == 2 && res.ContentLength == -1 {
+	// (see https://github.com/caddyserver/caddy/issues/3556 for use case and nuances)
+	if h.isBidirectionalStream(req, res) {
 		if wf, ok := rw.(http.Flusher); ok {
 			wf.Flush()
 		}