QNetworkReplyHttpImpl: convert another QDateTime::currentDateTime() to currentDateTimeUtc()

The latter is much faster as it doesn't have to deal with
time zones.

This change is safe because QNetworkHeadersPrivate::fromHttpDate(),
used as the QDateTime source in other, nearby, code paths, also
returns only UTC date-times. So not only is this change faster at
the call site, it also avoids inconsistent-timespec comparisons
down the line.

Credits to Milian Wolff, from whose QtWS15 talk this advice is
taken.

Change-Id: I26d308a2763cc45d28bc96871e651f30b17a6b85
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@theqtcompany.com>

src/network/access/qnetworkreplyhttpimpl.cpp

@@ -1585,7 +1585,7 @@ QNetworkCacheMetaData QNetworkReplyHttpImplPrivate::fetchCacheMetaData(const QNe
         QByteArray maxAge = cacheControl.value("max-age");
         if (!maxAge.isEmpty()) {
             checkExpired = false;
-            QDateTime dt = QDateTime::currentDateTime();
+            QDateTime dt = QDateTime::currentDateTimeUtc();
             dt = dt.addSecs(maxAge.toInt());
             metaData.setExpirationDate(dt);
         }