[BUGFIX beta] item/empty view should be lookuped from only global path

When `itemViewClass` is given the same name as global property name,
`CollectionView` couldn't look up registered view from container.

For example:

```handlebars
<!-- index.handlebars -->
{{view App.NamesView}}
```

```javascript
App.NameView = Ember.View.extend({
  template: Ember.Handlebars.compile('hi, {{view.content}}')
});

App.NamesView = Ember.CollectionView.extend({
  itemViewClass: "name",
  content: ['tomster']
});
```

Then I expect:
```html
hi, tomster
```

But actual, the assertion error (that is referenced global property
`window.name`) is thrown.