mod_rest: Workaround lack of number coercion in util.datamapper for HTTP upload util.datamapper will not coerce a string into an integer like the XEP-0363 'size' slot, which becomes a problem when using the HTTP GET method, passing fields as ?query parameters which are always strings.

This module blocks configured queries that change server state.

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

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