[BUGFIX beta] Calling a overrided method in builtin array, not change the enumerable properties

Given the following example:

```js
var array = [1];
array.objectAt(0);
Em.keys(array); // ["0", "__nextSuper"]
```

The `array` will end with the `__nextSuper` property, because when a overrided method is called, in that case `objectAt`, the `Ember.wrap` change the [`__nextSuper`](https://github.com/emberjs/ember.js/blob/master/packages/ember-metal/lib/utils.js#L335). 
That property is prevented to be enumerable in [`makeCtor`](https://github.com/emberjs/ember.js/blob/master/packages/ember-runtime/lib/system/core_object.js#L60), but not in a builtin array.