Body needs to be converted to a string if it is not already

[tdmalone]

eg. see tdmalone/coffee-shop-message#2, caused by a JSON object being sent through

[tdmalone]

This can of course be solved downstream by running JSON.stringify() before passing the message through, but as a JSON object is never an accepted value for body by API Gateway, we should ensure here that it doesn't get through. Of course, we just need to make sure that we're only stringify'ing if we have to, as we don't want to do it twice!