Improper use of time zone when checking for default values ​in adding a column

[zimulala]

Bug Report

Please answer these questions before submitting your issue. Thanks!

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

create table t(a int);
ALTER TABLE t ADD COLUMN ts timestamp DEFAULT '1970-01-01 08:00:01';

2. What did you expect to see?

mysql> select now();
+---------------------+
| now()               |
+---------------------+
| 2019-02-28 21:09:30 |
+---------------------+
1 row in set (0.00 sec)

mysql> create table t(a int);
Query OK, 0 rows affected (0.02 sec)

mysql> ALTER TABLE t ADD COLUMN ts timestamp DEFAULT '1970-01-01 08:00:01';
Query OK, 0 rows affected (0.03 sec)
Records: 0  Duplicates: 0  Warnings: 0

3. What did you see instead?

tidb> select now();
+---------------------+
| now()               |
+---------------------+
| 2019-02-28 21:09:20 |
+---------------------+
1 row in set (0.00 sec)

tidb> create table t(a int);
Query OK, 0 rows affected (0.01 sec)

tidb>  ALTER TABLE t ADD COLUMN ts timestamp DEFAULT '1970-01-01 08:00:01';
ERROR 1067 (42000): Invalid default value for 'ts'

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   Release Version: v3.0.0-beta-132-gddbf60f6d
   Git Commit Hash: ddbf60f
   Git Branch: master