Ability to create IncomingRabbitMQMetadata for testing purposes

[djarnis73]

When unit testing using InMemoryConnector I could not figure out a way to receive messages containing IncomingRabbitMQMetadata, since it does not have any public constructors. See my stack overflow post for more details.

Would it be possible to allow for manually creation of IncomingRabbitMQMetadata or should the InMemoryConnector translate the OutgoingRabbitMQMetadata into IncomingRabbitMQMetadata when the message is processed?

I would love to contribute a PR on this, but would like some guidance on the appropriate way to implement this.

Btw, I did manage to work around my immediate problem, by using mockito to mock IncomingRabbitMQMetadata, but I would prefer if this could be tested without mocking.