A delegate that a client can register that will be called each time a Producer's send method is
called to allow the client to Transform a sent message from one type to another, StreamMessage to
TextMessage, ObjectMessage to TextMessage containing XML, etc. This allows a client to create a
producer that will automatically transform a message to a type that some receiving client is
capable of processing or adding additional information to a sent message such as additional message
headers, etc. For messages that do not need to be processed the client should return null from
this method, in this case the original message will be sent.
function(session, producer, message);