CollectionProxy uses the arel of its association's scope.

CollectionProxy should be able to reuse the behavior (methods) of its parent class,
but with its own state. This change allows CollectionProxy to use the arel object
corresponding to its association's scope.
Merge pull request #14562 from jefflai2/collection_proxy

CollectionProxy uses the arel of its association's scope.

Conflicts:
	activerecord/CHANGELOG.md