Support force cache even when server doesn't set the Date header

This fixes a bug where when a upstream server doesn't set a Date
header the response will be cached but then once the cache is
expired it never gets refreshed because parseResponseHeaders
returns a no date header error

This also makes it so that the force_cache_duration_seconds
is respected when an upstream returns a 304 not modified

Signed-off-by: Peter <c2zwdjnlcg@users.noreply.github.com>