Prevent construction of unicode Message.data

And fix one case of it in parent.py.

mitogen/core.py

@@ -314,6 +314,7 @@ class Message(object):
         self.src_id = mitogen.context_id
         self.auth_id = mitogen.context_id
         vars(self).update(kwargs)
+        assert isinstance(self.data, str)
 
     def _unpickle_context(self, context_id, name):
         return _unpickle_context(self.router, context_id, name)

mitogen/parent.py

@@ -675,7 +675,7 @@ class RouteMonitor(object):
 
         data = str(target_id)
         if name:
-            data = '%s:%s' % (target_id, name)
+            data = '%s:%s' % (target_id, mitogen.core.b(name))
         self.parent.send(
             mitogen.core.Message(
                 handle=handle,