Export message to specified contentType via munge

        contentType <str> - eg. "json", "yaml"

def export(self, contentType):
        """
        Export message to specified contentType via munge

        contentType <str> - eg. "json", "yaml"
        """
        cls = munge.get_codec(contentType)
        codec = cls()
        return codec.dumps(self.__dict__())