Serialize on bulk calls. Serialize: this = full model.

- Serialize function now has the model as `this`.
- `cast` callback now has the entire model as `this`, so it can access another field:
```
...

es_cast: function() {
   return this.title;
}
...
```
- Bulk mode serializes too.

No new tests needed, this commit doesn't change functionality. All tests passed.