issue of foreign key cascade don't work

[crazycs520]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set @@global.tidb_enable_foreign_key=1;
set @@foreign_key_checks=1;
create table t1 (id int key);
create table t2 (id int key, a int, b int, foreign key fk_1 (a) references t1(id) ON DELETE SET NULL ON UPDATE SET NULL, foreign key fk_2 (b) references t1(id) ON DELETE CASCADE ON UPDATE CASCADE);
insert into t1 values (1), (2), (3);
insert into t2 values (1, 1, 1), (2, 2, 2), (3, 3, 3);
update t1 set id=id+10 where id in (1, 3);
delete from t1 where id = 2;

2. What did you expect to see? (Required)

> select * from t2;
+----+--------+----+
| id | a      | b  |
+----+--------+----+
| 1  | <null> | 11 |
| 3  | <null> | 13 |
+----+--------+----+

3. What did you see instead (Required)

> select * from t2;
+----+--------+---+
| id | a      | b |
+----+--------+---+
| 1  | <null> | 1 |
| 2  | <null> | 2 |
| 3  | <null> | 3 |
+----+--------+---+
3 rows in set

4. What is your TiDB version? (Required)

master: 0e6364f