fix tag names not case sensitive in mysql (fix #480)

This was because string comparison in mysql with utf8 is not case sensitive
by default.

mysql> SELECT COUNT(*) AS amount FROM tags WHERE tag='Général';
+--------+
| amount |
+--------+
|      1 |
+--------+
1 row in set (0.01 sec)

mysql> SELECT COUNT(*) AS amount FROM tags WHERE tag='général';
+--------+
| amount |
+--------+
|      1 |
+--------+
1 row in set (0.00 sec)