plain() removes my nested metadata from collections

I expected `plain()` to return the _real_ response, but it returns an object without metadata. (I extract my metadata as described in the [README](https://github.com/mgonto/restangular#my-response-is-actually-wrapped-with-some-metadata-how-do-i-get-the-data-in-that-case).)

It also has methods added within `setOnElemRestangularized` which I wouldn't expect. 
