Minor enhancement to stale-while-revalidate (RFC5861) handling; now we can also serve stale 304s (Not Modified) while asynchronously revalidating. Prior to this, we were always returning a stale 200 response regardless of whether the incoming request was conditional or not. The old behavior was not incorrect, but this is better.
httpcomponents/httpclient/trunk/httpclient-cache/src/main/java/org/apache/http/impl/client/cache/CachingHttpClient.java