MINOR: mux-h1: Consume channel's data in a loop in h1_snd_buf()

MINOR: mux-h1: Consume channel's data in a loop in h1_snd_buf()

In h1_snd_buf(), the data sending is done synchronously, as much as possible. So
if some data remains in the channel's buffer, because there was not enougth
place in the output buffer, it may be good the retry after a send because some
space may have been released when sending. Most of time the output buffer is
empty and all channel's data are consumed the first time. And if no data are
sent, we don't retry to do more. So the loop is just here to optimize edge cases
without any cost for all others.