[BUGFIX release] Allow Ember.Object.create to accept an Ember.Object.

Also, adds an assertion if the passed in value is not an object or
instance of `Ember.Object`.

Resolves #3596.
Merge pull request #3597 from rjackson/allow_ember_object_to_accept_an_ember_object

[BUGFIX release] Allow Ember.Object.create to accept an Ember.Object.