BUG/MEDIUM: stconn: Don't block sends if there is a pending shutdown

For the same reason than the previous patch, we must not block the sends
when there is a pending shutdown. In other words, we must consider the sends
are allowed when there is a pending shutdown.

This patch must slowly be backported as far as 2.2. It should partially fix
issue #2249.

include/haproxy/sc_strm.h

@@ -351,6 +351,8 @@ static inline int sc_is_send_allowed(const struct stconn *sc)
 {
 	if (sc->flags & SC_FL_SHUT_DONE)
 		return 0;
+	if (sc->flags & SC_FL_SHUT_WANTED)
+		return 1;
 
 	return !sc_ep_test(sc, SE_FL_WAIT_DATA | SE_FL_WONT_CONSUME);
 }