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.

This module produces more detailed HTTP logs for Prosodys built-in HTTP
server. The format is similar to that of Apache and go into Prosodys
normal logs at the `info` level.