Callback data

I get `null` when using:

```
rpc.call('say.Hello', { name: 'John' }, function(msg) {
  console.log(msg)
})
```

Even though msg is not really null, since this returns the stuff correctly:

```
rpc.call('say.Hello', { name: 'John' }, function() {
  console.log(arguments)
})
```
