Fix for DalliStore support

`.fetch_multi` wasn't working with DalliStore. Several tests were failing because the params passed to `IdentityCache.fetch_multi` were frozen strings and Dalli attempts to change their encoding (https://github.com/mperham/dalli/blob/master/lib/active_support/cache/dalli_store.rb#L275).

`fetch_multi` now gets passed the original array of cache keys instead of the keys to `key_to_id_map` (Hash keys are immutable strings).