authorize_resource is not called on the index action

[westonganger]

I was trying to implement my usual methods for CanCan but I was stumped for a bit why the authorize_resource method was never getting called no matter what I did.

Turns out that authorize_resource is never called on the index action which happened to be the page I was testing.

https://github.com/thoughtbot/administrate/blob/master/app/controllers/administrate/application_controller.rb#L5

I was expecting the method to called with the resource_class in place of the resource. I suppose this could pass the scoped_resource instead if that is preferred.

Would you accept a PR for this? If not then the documentation should be updated to include this little tidbit.