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 lets you block connections to any remote servers not on a
whitelist.

``` {.lua}
s2s_whitelist = {
    "example.org",
}
```