Really do not lose the default route when editing an interface

[bnc#883836#c15](https://bugzilla.novell.com/show_bug.cgi?id=883836#c15)
Well, "does not force default route on or off  ..." is the intended meaning. It can be also described "does not set DHCLIENT_SET_DEFAULT_ROUTE if it wasn't set". Would that be better?