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
Got, it. This is the same issue raised in #253 . The reason that is has not been resolved is that there is no clear answer on what to do when a class has multiple`belongs_to` associations. 

I strongly believe that this issue is outside of the scope of this pr.