Improve post.voted_by logic.
Improve post.voted_by logic.

nanaya/moebooru,moebooru/moebooru,nanaya/moebooru,euank/moebooru-thin,moebooru/moebooru,nanaya/moebooru,euank/moebooru-thin,nanaya/moebooru,moebooru/moebooru,euank/moebooru-thin,euank/moebooru-thin,euank/moebooru-thin,moebooru/moebooru,nanaya/moebooru,moebooru/moebooru