#2039 - Make admin/tags capital letter encoding more robust.

- change strtoupper() to mb_strtoupper().
For some locale settings, strtoupper() may not correctly capitalize the letters.
The cause of this is probably related to the comments in Gallery_I18n.php, where
it intentionally sets LC_CTYPE to 'C' for some cases.  Whatever the reason, the
solution is simple and insensitive to what the locale settings are.