fanout exchange should not require message type or routing key

[mxriverlynn]

It looks like wascally requires either a message type or routing key when using a fanout exchange type.

I should be able to produce and consume a message without either, when using a fanout type, as the fanout type completely ignores routing keys anyways.

I don't have any demo code at the moment, but I found this in some tests of my Rabbus library. I tried to publish a message with an empty string for the messageType and no routingKey, and it threw an error / didn't publish the message.

[arobson]

This feature was added to rabbot, the successor to wascally.