MCO-172 - fix subscription logic for rabbitmq.use_reply_exchange mode

This commit explains the reasoning behind the original
`return if type == :reply` gates in `#subscribe` and `#unsubscribe` and
the exception needed to them for the `use_reply_exchange` option.
Merge pull request #158 from richardc/mco-172

MCO-172 - fix subscription logic for rabbitmq.use_reply_exchange mode