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 blocks configured queries that change server state.

E.g. to make vCard storage read-only:

``` lua
readonly_stores = {
	vcard = { "vcard-temp", "vCard" };
}
```