This helps bring the interfaces of CollectionProxy and Relation closer
together, and reduces the delegation backflips we need to perform.
For example, first_or_create is defined thus:
first || create(...)
If CollectionProxy < Relation, then post.comments.first_or_create will
hit the association's #create method which will actually add the new record
to the association, just as post.comments.create would.
With the previous delegation, post.comments.first_or_create expands to
post.comments.scoped.first_or_create, where post.comments.scoped has no
knowledge of the association.