Fixing #923 - cookie expiry is now further in the past

On a loaded server, it's possible that the cookie expiry time may
actually end up being in the future compared to the date header.  This
puts it one day in the past.  This also should be long enough to deal
with any issues regarding timezones and clocks out of sync.
Merge pull request #923 from jroper/earlier-cookie-expiry

Cookie discard may not discard the cookie for the next request