don't call toJSON on models and collections

I'm not sure if this is really a good idea as it potentiall breaks a stuff ;) But, here's the problem I was facing:

I wanted to render a collection of ModelA, each using an existing view that can render that model. The view calls a helper method on `this.model`, and because of that the following collection view template fails to render because `this.model` is a JSON object in the view instantiated in the loop and not a model instance in the view:

```handlebars
{{#each models}}
    {{view "my_model_view" model=this}}
{{/each}}
```

Note that I did not change the behaviour of also returning `meta` and `models` although that would not break any tests and both of those are already part of the collection anyway. This seems a bit odd as well..