Fix anchor and focused on removal

I noticed that the `focused` and `anchor` observables weren't getting updated when the `foreach` observable array is manipulated from the outside. I added some tests that document my expectations and added a fix. I thought the `selection` array would also need to be updated (in case selected items are removed), but the tests I added for this already pass, so I didn't look further into that.

I could also do this housekeeping in my application, of course, but I hope this is within the scope of knockout.selection.