fixes an issue with has_many_through and polymorphic associations when source option is used.
fixes an issue with has_many_through and polymorphic associations when source option is used.

wvanbergen/scoped_search,lzap/scoped_search,jlsherrill/scoped_search,lzap/scoped_search,jlsherrill/scoped_search,wvanbergen/scoped_search,johnpmitsch/scoped_search,johnpmitsch/scoped_search