proxy-call: Adjust to the new callback always fired invariant (since d7d4d52)
proxy-call: Adjust to the new callback always fired invariant (since d7d4d52)

Since the callback is always fired when the call is completed, cancelled or an
error situation. We need to adapt the code for rest_proxy_call_cancel so that
it doesn't unref the call object a second time.

GNOME/librest,Distrotech/librest,ThomasBollmeier/librest-oauth-proxy,Distrotech/librest,ThomasBollmeier/librest-oauth-proxy,GNOME/librest,GNOME/librest