Support for optional async

Depends on this PR against rack-amqp-client: https://github.com/rack-amqp/rack-amqp-client/pull/1
Using NullResponse results in fewer needed conditionals
Make the tests happy!

Also, while !! is ugly, convert the async param to always be a boolean
to be more intent revealing in tests.
Merge pull request #9 from elight/async

Support for optional async