Interpolated text is not always projected correctly

This fails to project:

```
@Component({
  select: 'a'
})
@View({
  template: '<div><ng-content></ng-content></div>'
})
class A {}

Main template:
<a>{{1+2}}</a>
```

Reason: In the [ProtoViewMerger](https://github.com/angular/angular/blob/master/modules/angular2/src/render/dom/view/proto_view_merger.ts#L74) we don't add new `ng-binding` classes to elements into which bound elements are projected.
