mod_http_oauth2: Reject duplicate form-urlencoded parameters Per RFC 6749 section 3.1 > Request and response parameters MUST NOT be included more than once. Thanks to OAuch for pointing out Also cleans up some of the icky behavior of formdecode(), like returning a string if no '=' is included.

This module blocks configured queries that change server state.

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

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