BUG/MINOR: mux-h2: always send stream window update before connection's

BUG/MINOR: mux-h2: always send stream window update before connection's

In h2_process_mux() if we have some room and an attempt to send a window
update for the connection was pending, it's done first. But it's not done
for the stream, which will have for effect of postponing this attempt
till next pass into h2_process_demux(), at the risk of seeing the send
buffer full again. Let's always try to send both pending frames as soon
as possible.

This should be backported as far as 1.8.