BUG/MEDIUM: H2: Activate polling after successful h2_snd_buf().

Make sure h2_send() is called after h2_snd_buf() by activating polling.

This is 1.9-specific, no backport is needed.

src/mux_h2.c

@@ -3531,6 +3531,8 @@ static size_t h2_snd_buf(struct conn_stream *cs, struct buffer *buf, size_t coun
 	}
 
 	b_del(buf, total);
+	if (total > 0)
+		conn_xprt_want_send(h2s->h2c->conn);
 	return total;
 }