Messages from the opt-out worker fail to be routed

Typical error message:

```
No conversation info found for outbound message: <Message payload="{
    'transport_name': u'safaricom_sna_smpp_transport',
    'in_reply_to': u'f722a7d8-51b6-422a-ad74-45805105abd5',
    'group': None,
    'from_addr': u'XXXXX',
    'timestamp': datetime.datetime(2013, 6, 1, 6, 16, 19, 746126),
    'to_addr': u'+XXXXXXXXXXXX',
    'content': u'You have opted out',
    'routing_metadata': {u'endpoint_name': u'default'},
    ...
```