Model.clone() overwrites null attributes if default attribute value exists

The following seems like unexpected behavior and is causing some issues for us:

```js
var Model = Backbone.Model.extend({'a': 1});
var model = new Model();
model.set('a', null); // model.attributes is now { 'a': null }
var clone = model.clone(); // clone.attributes is now { 'a': 1 }
```