Check for JavaScript object types vs. Ember.Objects.

@dadah89 @vcekov @shajith @jamesarosen 

`Ember.Object`s can not be cloned, therefore we need to check to make sure the object we are dealing with is actually a JavaScript object.
Not sure if this "optimization" is even worth it. I just figured we'd be hitting the `isObject` path a lot and we could save on some string allocations & garbage collections. If it's confusing or superfluous I'm open to omitting it.