stale-if-error request cache directive should also apply to non-revalidatable cache entries

The stale-if-error request cache directive is defined in RFC 5861:

http://tools.ietf.org/html/rfc5861

As implemented, this will only apply to cache entries that are revalidatable (i.e. they have an Etag or Last-Modified header and can be refreshed with a conditional request) but it should also apply to any stale cache entry.