Added property middleware for persisting message properties into message...

... content.

 Copying from rabbitmq.com:

> Message properties

> The AMQP protocol predefines a set of 14 properties that go with a message. Most of the properties are rarely used, with the exception of the following:

> delivery_mode: Marks a message as persistent (with a value of 2) or transient (any other value). You may remember this property from the second tutorial.
> content_type: Used to describe the mime-type of the encoding. For example for the often used JSON encoding it is a good practice to set this property to: application/json.
> reply_to: Commonly used to name a callback queue.
correlation_id: Useful to correlate RPC responses with requests.

The `.property` middleware if used will add a `.properties` property in messages.