BUG/MINOR: cache: A 'max-age=0' cache-control directive can be overriden by a s-maxage

BUG/MINOR: cache: A 'max-age=0' cache-control directive can be overriden by a s-maxage

When a s-maxage cache-control directive is present, it overrides any
other max-age or expires value (see section 5.2.2.9 of RFC7234). So if
we have a max-age=0 alongside a strictly positive s-maxage, the response
should be cached.

This bug was raised in GitHub issue #2203.
The fix can be backported to all stable branches.