Fixed #23450 - transient failure of test_far_expiration

The cookie expiration date is now taken directly from
the expires parameter. However, I'm keeping the current
logic of adding 1 second to max_age, to ensure that expires
is never later than (now + max_age).
seems like the `else` branch could be removed and we can do `self.cookies[key]['expires'] = expires` no matter what.