Add support for delayed message exchange

[rept]

When using the rabbitmq_delayed_message_exchange plugin : https://blog.rabbitmq.com/posts/2015/04/scheduling-messages-with-rabbitmq

Exchanges are defined like this with Bunny:

exchange = channel.exchange('devices.delayed', type: 'x-delayed-message', durable: true)

This gave issues with the BunnyMock gem because the type 'x-delayed-message' isn't known as possible type for an exchange. This pull request adds support for that type.