core: Allow dead messages to be delivered regardless of policy

mitogen/core.py

@@ -1287,7 +1287,7 @@ class Router(object):
         if respondent:
             assert policy is None
             def policy(msg, _stream):
-                return msg.src_id == respondent.context_id
+                return msg.is_dead or msg.src_id == respondent.context_id
             def on_disconnect():
                 if handle in self._handle_map:
                     fn(Message.dead())