Scopes on associations

Punch a hole in the association_proxy for scopes. Also inject the parents id if needed.

Addresses https://github.com/remiprev/her/issues/215
I'm not sure exactly what your asking about. 

You don't have to explicitly set a collection path (https://github.com/remiprev/her/blob/master/lib/her/model/paths.rb#L48). If you don't set the collection_path it basically defaults to `/class_name`. 

However, if a user has many comments, and you need to get the comments through `/user/:user_id/comments` _Her_ requires a collection path to be set. There are some open issues discussing ways to deal with that and its far out side of the scope of this PR. 

If you're saying your suggesting that the right behavior is just to ignore whats in the collection path, I'd disagree since that would limit functionality.