`ClassSpecificRelation` has `method_missing` and the `method_missing` is
called first. if an associated class has the missing method in a
relation, never reach to the `method_missing` in the `CollectionProxy`.
I extracted `DelegateExtending` and included it to the delegate class
that including `ClassSpecificRelation` to fix the issue.

Fixes https://github.com/rails/rails/pull/28246#issuecomment-296033784.