Installer changes the case of MySQL usernames

Installer uses Kit's _USERNAME type to validate the MySQL usernames we get, however MySQL usernames are case sensitive so converting to lower case isn't helpful.

Work around as follows:
In install.php, on line 185, change it from this:
$db_user = Kit::GetParam('db_username',_POST,_USERNAME);
to
$db_user = Kit::GetParam('db_username',_POST,_PASSWORD);

and on line 192, change from this:
$db_admin_user = Kit::GetParam('admin_username',_POST,_USERNAME);
to
$db_admin_user = Kit::GetParam('admin_username',_POST,_PASSWORD);