Make the `all` method from test models return a `Relation`.
Make `Association` respond to `scope` in the models used for tests.
Add support scopes to be used in associations.
Add CHANGELOG entry.
No need to nodoc private methods.
Make `Relation` respond to `to_ary` and `each`, to avoid calling
`Model.all.to_a` in tests.
Merge pull request #1015 from plataformatec/add_support_to_scopes

Add support to association scope.