The scope of the MuleEvent Callback is to be able to get a the message currently
being processed by the
FunctionalTestComponent and make assertions
on the message payload, headers or attachments or to make changes required
for the test.
A test callback that writes the results of a service invocation to the response output stream
of the event
This should only be used when testing Asynchronous calls with the
FunctionalTestComponent otherwise
you will get duplicate messages, since both this class and the
FunctionalTestComponent will write
a return message back to the callee.