A previous commit removed some, but missed these. Where a location
directive was using Expires to set a future expiry in conjunction
with Cache-Control public, I have replaced the time with an equal
max-age.
Furthermore, Google's web performance guide says that "public" is
implicit if there is a max-age specified.
See: https://developers.google.com/web/fundamentals/performance/optimizing-content-efficiency/http-caching
Andy Dawson