mod_pubsub_post: Fix use of 'id' field in JSON object as pubsub item id `data` here is the source string, so there's no `id` field. Also only use the `id` field if it's a string.

module:set_global();

local log = _G.log;

module:add_timer(60-os.date("%S"), function (now)
	log("info", "-- MARK --");
	return 90 - ((now + 30) % 60);
end);