HTTPCLIENT-975: committed patch for stale-while-revalidate from Michajlo Matijkiw (michajlo_matijkiw at comcast dot com). Stale-while-revalidate functionality is currently off by default until we can add bounding to the revalidation queue (or make it configurable).
httpcomponents/httpclient/trunk/httpclient-cache/src/main/java/org/apache/http/impl/client/cache/AsynchronousValidationRequest.java