INT-3002 Fix Content-Type Mapping

The AMQP outbound endpoint behaves differently when configured
as a gateway when compared to an adapter.

With the adapter, the header mapper is invoked after the message
converter (via the MPP callback).

With the gateway, the mapper is invoked before the converter.

The SimpleMessageConverter unconditionally sets the content-type,
overriding any mapped content type.

Change the code to invoke the header mapper after the converter
to match the adapter logic.

Remove the TODO. Using a MPP in the gateway case does not provide
access to headers on the reply.

Add test cases.