Allow None in 'accept' parameter to Connection()

I'm consuming binary (Protocol Buffer) messages from the (@emile) RabbitMQ MQTT Adapter's `mqtt.topic` exchange. These come with a null (None) `content-type` for some reason (I've not looked into what's going on).

With my patch, I can cope with a null `content-type` by doing something like:

```python
identity = lambda x: x
serialization.register('raw', identity, identity, None)
# ...
Consumer(..., accept=[None], ...)
```

If there's a better way that renders this patch unnecessary, let me know. Thanks!