Added failing test for asynchronous use of '_super'

A [recent commit](https://github.com/emberjs/ember.js/commit/85b646bda98f15010df37c2094d89135f1a1a75f) broke some uses of `this._super` in my app. I don't know if it was intended behavior to be able to treat `this._super` as a closure, but I relied on being able to. The primary use case is for use with promises:

```javascript
DS.Model.reopen({
  save: function() {
    var save = this._super.bind(this);
    var otherRecords = [];

    // gather other records

    return Ember.RSVP.all(otherRecords.invoke('save')).then(save);
  }
});
```

Without knowing much about the internals of Ember's object model, I didn't feel well equipped enough to provide a proper fix.

EDIT: here's a [jsbin](http://emberjs.jsbin.com/oPupEHo/1) that demonstrates the issue.