Never broadcast without a namespace

https://github.com/reveal/multiplex/blob/master/client.js#L8

```javascript
// ignore data from sockets that aren't ours
if (data.socketId !== socketId) { return; }
```

Please, never do that : you're broadcasting messages for everybody, and ask clients for not listening what the others are shouting.

See https://socket.io/docs/v3/server-api/#server-of-nsp
