Fix `Ember.isArray` on IE8

The previous code couldn't check correctly.

``` javascript
// On IE8
Ember.isArray([]);
//=> false
```

`Ember.isArray` is used in critical point such as `Ember.Object.extend`.
So many tests failed to run.

This patch fixes some tests.
But many tests are broken yet.

Before:
![2014-05-22 0 31 16](https://cloud.githubusercontent.com/assets/290782/3044156/dc8ed1e8-e10e-11e3-9832-8477b0599b4b.png)

After:
![2014-05-22 1 49 33](https://cloud.githubusercontent.com/assets/290782/3044157/dc935556-e10e-11e3-962f-f13ec9335e33.png)

I'll work through on this one by one.