[BUGFIX beta] Support for mock dates in `Ember.typeOf`.

When mocking `Date` (using
[Timecop.js](https://github.com/jamesarosen/Timecop.js) for example),
`Ember.Enumerable.sortBy` would fail because of the type detection in
`Ember.compare` (`object` instead of `date`).

Conflicts:
	packages/ember-metal/lib/utils.js