BUG/MINOR: mux-h2: Don't pretend mux buffers aren't full anymore if nothing sent

BUG/MINOR: mux-h2: Don't pretend mux buffers aren't full anymore if nothing sent

In h2_send(), when something is sent, we remove the flags
(H2_CF_MUX_MFULL|H2_CF_DEM_MROOM) on the h2 connection. This way, we are able to
wake up all streams waiting to send data. Unfortunatly, these flags are
unconditionally removed, even when nothing was sent. So if the h2c is blocked
because the mux buffers are full and we are unable to send anything, all streams
in the send_list are woken up for nothing. Now, we only remove these flags if at
least a send succeeds.

This patch must be backport to 2.0.