Auto clean selection after content of foreach is altered

This pull is for passing two failed tests introduced in https://github.com/mwoc/knockout.selection/commit/d25a38ef5e2f29af64c9544907da34f62e8b55f1
It also makes the test "has no selection after the selected item is removed from the observable array" pass - which I think worked before but in the current (bramstein:master) fails.

The only failing test left if this pull is accepted, is: "empties its selection when the observableArray bound to the foreach changes".