@_cancan_skipper should be inherited: add failing test

If a superclass controller has:

    load_and_authorize_resource
    skip_load_resource :only => :index

A subclass will not skip the loading of the resource on the `index` action.

I've added a failing test to illustrate this behavior.

Potential fix would be to use `class_attribute` from Rails: http://apidock.com/rails/Class/class_attribute

Moved from https://github.com/ryanb/cancan/pull/683