Really do not lose the default route when editing an interface

[bnc#883836#c15](https://bugzilla.novell.com/show_bug.cgi?id=883836#c15)
No, I don't use it both ways. There is an important distinction in the other direction where everything else than yes/no becomes nil. Also, there is no `Hash#reverse`.
Yes, I will use a constant here.