Backbone.Subset does not correctly extend Backbone.Collection

If you create a new instance of a Backbone.Subset, it does not act as an instance of a Backbone.Collection:

!(mySubset instanceof Backbone.Collection) === true

This is a problem because it means you can't always substitute a Subset for a Collection.  Knockback, for instance, can't make a collectionObservable around a Subset because it does an instanceof safety check.
