[Fix #90] Allow for listening to both text and byte messages

[slipset]

We have a use case at work where we listen to both text and byte messages. This pre disallows that. I'm not sure if there is a reason for disallowing, so please close if this is not a good idea.

Looking back into the old issue that introduced this assertion, the underlying problem might now just be fixed because now there is only one listener created, and thus close should only be called once?