Fixed bug in ActiveFedora::FinderMethods#load_from_fedora so that it casts
when klass is ActiveFedora::Base and `cast' argument is nil, as intended.
Fixes #431 since by default it calls load_from_fedora(pid, nil) on each hit.
Merge pull request #434 from dchandekstark/find-each-cast

Fixed bug in ActiveFedora::FinderMethods#load_from_fedora