mod_http_oauth2: Simplify template using if-falsy operator Relies on Prosody rev af1e3b7d9ea3 which added the {var~if-falsy}, released in 0.12. Since this module requires trunk this is fine.

This module blocks configured queries that change server state.

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

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