Try using catchAll:
robot.catchAll (msg) ->
msg.send msg.message.text
However it will match a message that no other matchers matched. If you want to catch absolutely everything, you'll have to do this:
robot.respond /(.*)/, (msg) ->
msg.send msg.match[1]
Update:
Question was how to get the original message, not how to catch all messages. Answer:
robot.respond /test/, (msg) ->
msg.send msg.message.text