Cache: send conditional requests only for cached 200/206 responses. RFC7232 says: The 304 (Not Modified) status code indicates that a conditional GET or HEAD request has been received and would have resulted in a 200 (OK) response if it were not for the fact that the condition evaluated to false. which means that there is no reason to send requests with "If-None-Match" and/or "If-Modified-Since" headers for responses cached with other status codes. Also, sending conditional requests for responses cached with other status codes could result in a strange behavior, e.g. upstream server returning 304 Not Modified for cached 404 Not Found responses, etc. Signed-off-by: Piotr Sikora <piotr@cloudflare.com>