Fetch many instances by id on a single request.

Some times you'd like to find many instances by id
with a single request. Using `where(id: [1, 2])` is
most likely the way rails people would try to do this.
So I just fixed Her to treat `id` as yet another
request param if its value turns out to be an array.
Merge pull request #191 from vic/find_many_by_id_single_request

Fetch many instances by id on a single request.