Fix for {{view class=someBoundProp}}.

Error happens in [packages/ember-handlebars/lib/helpers/view.js#L83](https://github.com/emberjs/ember.js/blob/master/packages/ember-handlebars/lib/helpers/view.js#L83), which is:

```javascript
    if (hash.classBinding) {
      extensions.classNameBindings = hash.classBinding.split(' ');
    }
```

Where `classBinding` there is a `StreamBinding` instance (which does not have a `split` method).

---

**Update:** Whitelist `classBinding` to allow it to be transformed into a stream later.
nit pick, `value = hash[prop]` is defined above, may as well use it?