redbot.org acts like lint for http headers. It identified an issue where Last-Modified header
was returning a date with a timezone of -0000. It should have been GMT to match HTTP spec.

Fixed this in changeset:   8018:67922fd17454.

However it is also finding issues with Vary header where Accept-Encoding is not always
added. In cases where encoding is used (gzip, br) the Accept-Encoding value is added.
However when no encoding is used, the Accept-Encoding value is not added. It should be
present to allow the cache to uniquely id the unencoded case according to redbot.

It also reports that requests for a resource done with If-None-Match If-Modified-Since
headers return the full file contents rather than returning an empty body with a 304 not
modified header. I need to debug this case because it might be my caching proxy showing
the issue and not the roundup-server.

Check for other issues as well by hitting different endpoints.