Really do not lose the default route when editing an interface

[bnc#883836#c15](https://bugzilla.novell.com/show_bug.cgi?id=883836#c15)
I'd prefer squashed form or one "dot block" per line

this one:
```
allow(Yast::NetworkInterfaces)
  .to receive(:GetTypeFromSysfs)
  .with(/eth\d+/)
  .and_return "eth"
```
is my favorite ;-)