select cast(111 as decimal(1)) is not consistent with MySQL

[XuHuaiyu]

Please answer these questions before submitting your issue. Thanks!

1. What did you do?
   If possible, provide a recipe for reproducing the error.

select cast(111 as decimal(1));

2. What did you expect to see?

mysql> select cast(111 as decimal(2));
+-------------------------+
| cast(111 as decimal(1)) |
+-------------------------+
|                      9 |
+-------------------------+
1 row in set, 1 warning (0.00 sec)

3. What did you see instead?

mysql> select cast(111 as decimal(1));
+-------------------------+
| cast(111 as decimal(1)) |
+-------------------------+
|                     111 |
+-------------------------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V)?

Git Commit Hash: 2198d0e7521a5c6c493a4c7af58509b6b277fd6e
UTC Build Time:  2017-05-23 03:07:15