mod_unified_push: Allow additional CORS headers for unified push (fixes #1985) - Request header "TTL" is mandated by the HTTP Push specification RFC 8030: https://datatracker.ietf.org/doc/html/rfc8030#section-5 - Firefox sends "Content-Encoding" with a HTTP form submission, so it is required when using the Unified Push testing tool at https://unifiedpush.org/test_wp.html (with the full URL being delivered by the UP-Example Android app) Adding both headers makes the Unified Push testing tool work from the browser. Resolves: https://issues.prosody.im/1985

# 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