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 how this bug can strike in
practice.

Signed-off-by: Amir Hadzic <amir.hadzic@randomshouting.com>
Switch from @each to []

Signed-off-by: Amir Hadzic <amir.hadzic@randomshouting.com>