MEDIUM: stream-interface: the polling flags must always be updated in chk_snd_conn

We used to only update the polling flags in data phase, but after that
we could update other flags. It does not seem possible to trigger a
bug here but it's not very safe either. Better always keep them up to
date.
diff --git a/src/stream_interface.c b/src/stream_interface.c
index abbbcb1..bb5b962 100644
--- a/src/stream_interface.c
+++ b/src/stream_interface.c
@@ -980,15 +980,13 @@
 		return;
 	}
 
-	if (!(conn->flags & (CO_FL_HANDSHAKE|CO_FL_WAIT_L4_CONN|CO_FL_WAIT_L6_CONN))) {
-		/* Before calling the data-level operations, we have to prepare
-		 * the polling flags to ensure we properly detect changes.
-		 */
-		if (conn_ctrl_ready(conn))
-			fd_want_send(conn->t.sock.fd);
-
-		conn_refresh_polling_flags(conn);
+	/* Before calling the data-level operations, we have to prepare
+	 * the polling flags to ensure we properly detect changes.
+	 */
+	conn_refresh_polling_flags(conn);
+	__conn_data_want_send(conn);
 
+	if (!(conn->flags & (CO_FL_HANDSHAKE|CO_FL_WAIT_L4_CONN|CO_FL_WAIT_L6_CONN))) {
 		si_conn_send(conn);
 		if (conn_ctrl_ready(conn) && (conn->flags & CO_FL_ERROR)) {
 			/* Write error on the file descriptor */