mod_http_oauth2: Reject invalid attempt to register client without credentials The implicit flow works without a client_secret since the token is delivered directly, but all other currently supported grant types require client to authenticate using credentials, so it makes no sense to not issue credentials then.

This module blocks configured queries that change server state.

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

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