[BUGFIX beta] Object.create(null) in Ember.inspect

This commit resolves an issue where objects passed to Ember.inspect that
contained an `Object.create(null)` object as a property would cause an
exception to be thrown.

I hit this in my app because we were `Ember.inspect`ing objects (a model
in this case) that had their container property set. The container’s
registry as of 1.8.0 is (you guessed it) an object with a null prototype.
Merge pull request #5577 from emberjs/inspect-null-prototype

[BUGFIX beta] Object.create(null) in Ember.inspect