Fixed bug in acl introduced by acca10b6abe74b3ab51890d5cbe0ab8da4fdf7e0

See comment in https://github.com/zendframework/zf2/commit/acca10b6abe74b3ab51890d5cbe0ab8da4fdf7e0#library/Zend/Permissions/Acl/Acl.php