Minor Description
Currently stale-while-revalidate mechanism works only if the cache entry contains a last-modified or a etag header. It should work also for cache entries with expires or max-age.
The specification is not very clear about what a revalidation is, but in the example there is no last-modified nor etag, only a cache-control: max-age
http://tools.ietf.org/html/rfc5861
If everybody agrees, I can work on a fix.