fix: don't change the receive window if we're forcing an update

Otherwise, our initial SYN/SYN-ACK packet will update the receive window
every time.

session_test.go

@@ -1195,7 +1195,6 @@ func TestSession_PartialReadWindowUpdate(t *testing.T) {
 		sendWindow := atomic.LoadUint32(&wr.sendWindow)
 		if sendWindow != initialStreamWindow {
 			t.Errorf("sendWindow: exp=%d, got=%d", client.config.InitialStreamWindowSize, sendWindow)
-			return
 		}
 
 		n, err := wr.Write(make([]byte, flood))

stream.go

@@ -214,8 +214,9 @@ func (s *Stream) sendWindowUpdate() error {
 	if !needed {
 		return nil
 	}
+
 	now := time.Now()
-	if rtt := s.session.getRTT(); rtt > 0 && now.Sub(s.epochStart) < rtt*4 {
+	if rtt := s.session.getRTT(); flags == 0 && rtt > 0 && now.Sub(s.epochStart) < rtt*4 {
 		var recvWindow uint32
 		if s.recvWindow > math.MaxUint32/2 {
 			recvWindow = min(math.MaxUint32, s.session.config.MaxStreamWindowSize)