Ember.Select breaks when content has changed and value hasn't

Changing `Ember.Select`'s `content` should invalidate the `selection` property. Otherwise, you can end up with a stale `selection` that is not in `content`, which causes the SelectedOptions to fail to match.

The included unit test demonstrates one way this bug can strike in practice: swapping out models that each contain their own selection choices, with overlapping values.