ISPN-4867 Single event marshaller support more clearly defined in API

* Based on Dan's feedback, Hot Rod server API made it look as if you could pass a per-factory marshaller but that's not really the case. A single event marshaller is currently supported.
In fact it should be:
``` 
private def selectMarshaller = marshaller.orElse(defaultMarshaller)
```
in order to keep the same return value as the original method