=== AmqpForwardAttribute

This mailet forwards the attributes values to a AMPQ.

It takes 4 parameters:

* attribute (mandatory): content to be forwarded, expected to be a Map<String, byte[]>
where the byte[] content is issued from a MimeBodyPart.
It is typically generated from the StripAttachment mailet.
* uri (mandatory): AMQP URI defining the server where to send the attachment.
* exchange (mandatory): name of the AMQP exchange.
* routing_key (optional, default to empty string): name of the routing key on this exchange.

This mailet will sent the data attached to the mail as an attribute holding a map.