if property in template data is a model, setting it to null will break reset method

Let's say I have a Ractive view of

```javascript
var view = new Ractive.extend({
    template: template,
    data: {
        content: new Backbone.Model.extend()
    }
});
```

If later in my application I decide to execute `view.set('content', null)`, this will cause an error in the reset method of the `BackboneModelWrapper`.  Since `null` is described as an `Object` in JavaScript, the current `typeof` check will fail.  This PR updates the `typeof` check to use `instanceof` for better type checking accuracy.