[RabbitMQ] Support for the default exchange

[rafaelelter]

What would you like to see added?

Support for RabbitMQ's default exchange (i.e. ""). Currently the application returns an error: exchange name cannot be empty.

Use Case

RabbitMQ's default exchange is still pretty common in small projects.

[blinktag]

@rafaelelter excellent idea! I will work on getting default exchanges working in plumber shortly