fix network caching code for PreferNetwork

diff --git a/src/network/access/qnetworkaccesshttpbackend.cpp b/src/network/access/qnetworkaccesshttpbackend.cpp
index 44afd4d..2d5497f 100644
--- a/src/network/access/qnetworkaccesshttpbackend.cpp
+++ b/src/network/access/qnetworkaccesshttpbackend.cpp
@@ -398,7 +398,10 @@ void QNetworkAccessHttpBackend::validateCache(QHttpNetworkRequest &httpRequest,
     if (lastModified.isValid())
         httpRequest.setHeaderField("If-Modified-Since", QNetworkHeadersPrivate::toHttpDate(lastModified));
 
-    if (CacheLoadControlAttribute == QNetworkRequest::PreferNetwork) {
+    if (CacheLoadControlAttribute == QNetworkRequest::PreferNetwork)
+        return; // PreferNetwork == send the request on the network with a "If-Modified-Since" header, if possible
+
+    if (CacheLoadControlAttribute == QNetworkRequest::PreferCache) {
         it = cacheHeaders.findRawHeader("Cache-Control");
         if (it != cacheHeaders.rawHeaders.constEnd()) {
             QHash<QByteArray, QByteArray> cacheControl = parseHttpOptionHeader(it->second);