mod_http_oauth2: Strip unknown extra fields from client registration We shouldn't sign things we don't understand! RFC 7591 section-2 states: > The authorization server MUST ignore any client metadata sent by the > client that it does not understand (for instance, by silently removing > unknown metadata from the client's registration record during > processing). Prevents grandfathering in of unvalidated data that might become used later, especially since the 'additionalProperties' schema keyword was removed in 698fef74ce53

# Introduction

This module sends a [version query][xep0092] to clients when they
connect, and logs the response, allowing statistics of client usage to
be recorded.

Note that since this is per connection, there will be a bias towards
mobile clients on bad connections.

## Example

    info    Running Running Swift version 4.0