Strip loose routes when forwarding upstream

Strip out `Route:` headers on all messages proxied from Bono to Sprout.  In particular, the UE might (should) have added a `Route:` header matching the `Service-Route:` we send back on the `REGISTER` response (see #42).

In any case, if we're proxying the message up to Sprout, we're taking ownership of the message's route and shouldn't allow loose-routing.