MINOR: mux_h1: Set H1_F_CO_MSG_MORE if we know we have more to send.

MINOR: mux_h1: Set H1_F_CO_MSG_MORE if we know we have more to send.

In h1_snd_buf(), also set H1_F_CO_MSG_MORE if we know we still have more to
send, not just if the stream-interface told us to do so. This may happen if
the last block of a transfer doesn't fit in the buffer, it remains useful
for the transport layer to know that more data follows what's already in
the buffer.