votingapi_set_votes needs to pass back new vote objects

When using votingapi_set_votes, the update/inserted vote object need to be returned so that affected vote_ids are available to the calling module. This change is already in place in votingapi_add_votes(), it just needs to be replicated in the set votes function also.

This patch just make the $votes parameter passed by reference (like votingapi_add_votes).