add test for find/where through association issue in #303.
use a module (QueryMethods) rather than a class (Relation) so that we can re-use the same methods when patching CollectionAssociation.
Include QueryMethods in delegated subclasses of ActiveRecord::Relation for
translated model. This enables translated query methods on associations, as
discussed in #303.
Check that delegated relation classes are defined before including
QueryMethods from them, to safeguard against changes in ActiveRecord.
super returns class that includes globalize query methods, so we can call with_translations_in_fallbacks.
Merge pull request #305 from globalize/issue_303_find_through_association

Fixes #303.