is_alive() on ActorProxy may cause ActorDeadError

[jodal]

Using Pykka 0.12.2.

1. Get a proxy for a running actor.
2. ActorRegistry.stop_all()
3. Call proxy.actor_ref.is_alive() results in the following exception:

Traceback (most recent call last):
  File "/home/jodal/dev/mopidy/tests/frontends/mpris/root_interface_test.py", line 17, in tearDown
    if self.backend.actor_ref.is_alive():
  File "/usr/lib/pymodules/python2.6/pykka/proxy.py", line 81, in __getattr__
    self._update_attrs()
  File "/usr/lib/pymodules/python2.6/pykka/proxy.py", line 61, in _update_attrs
    {'command': 'pykka_get_attributes'})
  File "/usr/lib/pymodules/python2.6/pykka/actor.py", line 426, in send_request_reply
    self.send_one_way(message)
  File "/usr/lib/pymodules/python2.6/pykka/actor.py", line 394, in send_one_way
    raise _ActorDeadError('%s not found' % self)
ActorDeadError: DummyBackend (urn:uuid:0d3a1518-0d19-48ef-b691-b359f177bcda) not found