make options optional like default Backbone.sync

Hi,

The options parameter from the default Backbone.sync method is optional. So if an app use Backbone.sync("update", my_collection), adding this library will break it. This pull request simply add one line which make it work again.

edit: For reference, here is where the backbone.sync method make the default options http://backbonejs.org/docs/backbone.html#section-132