set character_set_results = null seems not work

[jackysp]

Please answer these questions before submitting your issue. Thanks!

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

set character_set_results = null;
show variables where variable_name='character_set_results';

2. What did you expect to see?

mysql> set character_set_results = null;
Query OK, 0 rows affected (0.03 sec)

mysql> show variables where variable_name='character_set_results';
+-----------------------+-------+
| Variable_name         | Value |
+-----------------------+-------+
| character_set_results |       |
+-----------------------+-------+
1 row in set (0.11 sec)

3. What did you see instead?

mysql> set character_set_results = null;
Query OK, 0 rows affected (0.00 sec)

mysql> show variables where variable_name='character_set_results';
+-----------------------+--------+
| Variable_name         | Value  |
+-----------------------+--------+
| character_set_results | latin1 |
+-----------------------+--------+
1 row in set (0.00 sec)

4. What version of TiDB are you using (tidb-server -V or run select tidb_version(); on TiDB)?
   Git Commit Hash: 7e48ab1
   Git Branch: master
   UTC Build Time: 2018-08-06 08:15:07
   GoVersion: go version go1.10.3 darwin/amd64
   Race Enabled: false
   TiKV Min Version: 2.1.0-alpha.1-ff3dd160846b7d1aed9079c389fc188f7f5ea13e