Upstream: cache revalidation with conditional requests. The following new directives are introduced: proxy_cache_revalidate, fastcgi_cache_revalidate, scgi_cache_revalidate, uwsgi_cache_revalidate. Default is off. When set to on, they enable cache revalidation using conditional requests with If-Modified-Since for expired cache items. As of now, no attempts are made to merge headers given in a 304 response during cache revalidation with headers previously stored in a cache item. Headers in a 304 response are only used to calculate new validity time of a cache item.