Return self from CollectionDecorator#replace

This is necessary when storing CollectionDecorators in a HashWithIndifferentAccess, because of the way it [recursively converts values](https://github.com/rails/rails/blob/7cc26fd15e27c4a13705a844538bebfdd0461729/activesupport/lib/active_support/hash_with_indifferent_access.rb#L169-L170).

Closes #516.