failing test for polymorphic relationship being set up correctly

Maybe a better data example is this:
```
Contact = DS.Model.extend({
  posts: DS.hasMany( 'post')
});

Email = Contact.extend();
Telephone = Contact.extend();

Post = DS.Model.extend({
 contact: belongsTo('contact', { polymorphic: true})
});
```

I should be able to create a post passing in an email or telephone as contact.