V2.0.12 Fix set global variables error

[alpes214]

Description:
Fixes the issue #2741

Test:
set statement changes default_% variables as expected without an error.

mysql> select * from global_variables where variable_name like '%character%';
+----------------------------------------+----------------+
| variable_name                          | variable_value |
+----------------------------------------+----------------+
| mysql-default_character_set_results    | utf8mb4        |
| mysql-default_character_set_connection | utf8mb4        |
| mysql-default_character_set_client     | utf8mb4        |
| mysql-default_character_set_database   | utf8mb4        |
+----------------------------------------+----------------+
4 rows in set (0.00 sec)

mysql> set mysql-default_character_set_client='utf8';
Query OK, 1 row affected (0.00 sec)

mysql> set mysql-default_character_set_connection='utf8';
Query OK, 1 row affected (0.00 sec)

mysql> select * from global_variables where variable_name like '%character%';
+----------------------------------------+----------------+
| variable_name                          | variable_value |
+----------------------------------------+----------------+
| mysql-default_character_set_results    | utf8mb4        |
| mysql-default_character_set_connection | utf8           |
| mysql-default_character_set_client     | utf8           |
| mysql-default_character_set_database   | utf8mb4        |
+----------------------------------------+----------------+
4 rows in set (0.00 sec)

[renecannao]

It looks good to me.
Please create a similar PR for 2.1