Alter behavior of read_multi to match ActiveSupport::Cache::Store

In the base implementation of ActiveSupport::Cache::Store (and all other
implementing stores), an array of strings passed as argument to
read_multi is treated as a single key. In DalliStore, it is treated as
several keys. This alters the spec and implementation of DalliStore's
read_multi to match ActiveSupport::Cache::Store.
Merge pull request #362 from macreery/read_multi-fix

Alter behavior of read_multi to match ActiveSupport::Cache::Store