o ignore
    'proxy-invalidate' since we aren't a proxy.
    We will never return a stale document as
    fresh as a design decision, and thus the non-implementation
    of 'max-stale'. This also lets us safely ignore 'must-revalidate'
    since we operate as if every server has sent 'must-revalidate'.
    Since we are private we get to ignore both 'public' and
    'private' parameters. We also ignore 'no-transform' since
    we don't do any transformations.
    The 'no-store' parameter is handled at a higher level.
    So the only Cache-Control parameters we look at are:

    no-cache
    only-if-cached
    max-age
    min-fresh
    Ú