voter_type and votable_type are always base_class

If one voted for STI'ed model and then called `liked?` on it, it was always `false`, because Vote was implicitly saved with `base_class` in `_type` fields here https://github.com/ryanto/acts_as_votable/blob/master/lib/acts_as_votable/votable.rb#L87, but queried with the subclass condition.