LDAP: cachekey in set method needs to match with that one from get

$cacheky in https://github.com/owncloud/core/blob/master/apps/user_ldap/lib/access.php#L1015 needs to match with that in https://github.com/owncloud/core/blob/master/apps/user_ldap/lib/access.php#L1034, it won't ever work like it is now.

another low-hanging review, @bartv2 @karlitschek, @Raydiation or others please.