python: Don't override message_schema for json schemas

[gasmith]

I watched @amacneil today as he did this:

chan = Channel("topic", message_encoding="msgpack", schema={ ... })

That doesn't work as expected, because we automatically override the message_encoding to be json when you provide a schema that looks like a dict jsonschema. Which is a silly thing to do, in retrospect. The schema encoding does not imply a message encoding.