can't filter on nested data

```
this works
    return rethinkdbAdapter.r.table('limits').filter({'user': {'user_id': user_id}}).run();
// this does not
    return Limit.findAll({where: {'user.user_id': {'==': user_id}}});
// this does not work either
    return Limit.findAll({'user.user_id': user_id});
```
