BUG/MEDIUM: streams: Don't switch the SI to SI_ST_DIS if we have data to send.

In sess_established(), don't immediately switch the backend stream_interface
to SI_ST_DIS if we only got a SHUTR. We may still have something to send,
ie if the request is a POST, and we should be switched to SI_ST8DIS later
when the shutw will happen.

This should be backported to 2.0 and 1.9.
diff --git a/src/stream.c b/src/stream.c
index 80120d0..b22eef2 100644
--- a/src/stream.c
+++ b/src/stream.c
@@ -944,8 +944,9 @@
 		si_chk_rcv(si);
 	}
 	req->wex = TICK_ETERNITY;
-	/* If we managed to get the whole response, switch to SI_ST_DIS now. */
-	if (rep->flags & CF_SHUTR)
+	/* If we managed to get the whole response, and we don't have anything
+	 * left to send, or can't, switch to SI_ST_DIS now. */
+	if (rep->flags & (CF_SHUTR | CF_SHUTW))
 		si->state = SI_ST_DIS;
 }