Fix bug when `defaults` has a collection.

[mittsh]

When defaults set the value as an empty collection, it was using the collection as the only item of a new collection, e.g.:

        Book = Backbone.Model.extend({
            relations: {
              author: Author,
              pages: Backbone.Collection
            },
            defaults: {
              author: new Author(),
              pages: new Backbone.Collection()
            }
        });

What we expect is that the empty collection is kept.

Adding unit tests for that in defaults.js, passing now, breaking before this change.

[blittle]

Thx for the pull req, released in v2.0.2

[mittsh]

@blittle :+1: awesome!