issue #413: don't double-propagate DEL_ROUTE to parent.

propagate_up() sends ADD_ROUTE and DEL_ROUTE

propagate_down() sends only DEL_ROUTE, but didn't bother checking if
up() had sent it already.

Fixes:

     ERROR! [pid 41060] 17:55:30.739159 E mitogen.ctx.ssh.localhost:
     mitogen: RouteMonitor(): received DEL_ROUTE for 6081 from
     mitogen.fork.Stream(u'fork.41142'), expected
     mitogen.core.Stream('parent')

mitogen/parent.py

@@ -1780,7 +1780,10 @@ class RouteMonitor(object):
             ID of the connecting or disconnecting context.
         """
         for stream in self.router.get_streams():
-            if target_id in stream.egress_ids:
+            if target_id in stream.egress_ids and (
+                    (self.parent is None) or
+                    (self.parent.context_id != stream.remote_id)
+                ):
                 self._send_one(stream, mitogen.core.DEL_ROUTE, target_id, None)
 
     def notice_stream(self, stream):