Use lowercase indices when traversing ldap_get_entries result

As per http://www.php.net/manual/en/function.ldap-get-entries.php
ldap_get_entries converts indices to lowercase in its return value,
so the same was done in GenericLDAPServer->getUserAttributes() when
$fname/$sname/$mail_tag were being used as indices to the $data
array returned by ldap_get_entries.