BUG/MINOR: httpclient: CF_SHUTW_NOW should be tested with channel_is_empty()

CF_SHUTW_NOW shouldn't be a condition alone to exit the io handler, it
must be tested with the emptiness of the response channel.

Must be backported to 2.5.

src/http_client.c

@@ -928,7 +928,8 @@ static void httpclient_applet_io_handler(struct appctx *appctx)
 	 * isn't any data to handle and a shutdown is detected, let's stop
 	 * everything */
 	if ((req->flags & (CF_SHUTR|CF_SHUTR_NOW)) ||
-	    (res->flags & (CF_SHUTW|CF_SHUTW_NOW))) {
+	    (res->flags & CF_SHUTW) ||
+	    ((res->flags & CF_SHUTW_NOW) && channel_is_empty(res))) {
 		goto end;
 	}
 	return;