Qt clobbers the cache-control headers set by WebCore when CacheLoadControlAttribute == QNetworkRequest::AlwaysNetwork

Qt clobbers the cache-control headers set by WebCore when CacheLoadControlAttribute == QNetworkRequest::AlwaysNetwork. This causes at least one layout test to fail: http/tests/misc/refresh-headers.php

Qt needs to detect if the client has already made a cache-control decision in the headers before overwriting them.