api/resource_collection.rb needs size and empty? methods

Hello again. We have existing code that checks api responses for empty collections ( one example is `account.sources` ). The code as currently written does not allow one to ask a resource collection if it is empty or it's size without iterating over the collection using `each` or one of the included methods from `Enumerable`.

I understand that loading the collections would need to be forced to implement these methods but, in our case, we are ok with that. Perhaps a not in the api docs for the methods like:

    Calling this method will force the collection to be loaded

thoughts?