[BUGFIX beta] Ensure context is unchanged when using keywords with itemController.

Prior to this change the following code would bind `this` in the template block to the itemController's content (even though the keyword form of `{{each}}` is used).

This change sets the child view's `_context` property to the current context when using the keyword form of `{{each}}`, and a couple of confirming tests to demonstrate using `itemController` specified in either the `ArrayController` or the template directly.

Failing JSBin: http://emberjs.jsbin.com/zokali/1/edit.

Passing JSBin: http://emberjs.jsbin.com/kecen/1/edit.

Fixes #4634.