added ability to paginate and use sort order in CSV

This adds the ability to use the sort order in the CSV output. Resolves issue https://github.com/activeadmin/activeadmin/issues/3269
sorry, forget to answer. Yes, apply this https://github.com/activeadmin/activeadmin/pull/3375#discussion_r16935635 with the `@collection` change and it would nice to see some more tests