QHttpHeaders: Self-Encapsulate Field d->headers

Use QHttpHeaders::reserve() and QHttpHeaders::isEmpty() instead of
going directly to d->headers. Will help in enabling a nullptr d.

Pick-to: 6.7
Change-Id: Id530f8922b17058ec47530523ed43e08927c3ce3
Reviewed-by: Juha Vuolle <juha.vuolle@qt.io>

src/network/access/qhttpheaders.cpp

@@ -485,7 +485,7 @@ QHttpHeaders::QHttpHeaders() : d(new QHttpHeadersPrivate)
 QHttpHeaders QHttpHeaders::fromListOfPairs(const QList<std::pair<QByteArray, QByteArray>> &headers)
 {
     QHttpHeaders h;
-    h.d->headers.reserve(headers.size());
+    h.reserve(headers.size());
     for (const auto &header : headers)
         h.append(header.first, header.second);
     return h;
@@ -500,7 +500,7 @@ QHttpHeaders QHttpHeaders::fromListOfPairs(const QList<std::pair<QByteArray, QBy
 QHttpHeaders QHttpHeaders::fromMultiMap(const QMultiMap<QByteArray, QByteArray> &headers)
 {
     QHttpHeaders h;
-    h.d->headers.reserve(headers.size());
+    h.reserve(headers.size());
     for (const auto &[name,value] : headers.asKeyValueRange())
         h.append(name, value);
     return h;
@@ -515,7 +515,7 @@ QHttpHeaders QHttpHeaders::fromMultiMap(const QMultiMap<QByteArray, QByteArray>
 QHttpHeaders QHttpHeaders::fromMultiHash(const QMultiHash<QByteArray, QByteArray> &headers)
 {
     QHttpHeaders h;
-    h.d->headers.reserve(headers.size());
+    h.reserve(headers.size());
     for (const auto &[name,value] : headers.asKeyValueRange())
         h.append(name, value);
     return h;
@@ -1097,7 +1097,7 @@ QMultiHash<QByteArray, QByteArray> QHttpHeaders::toMultiHash() const
 */
 void QHttpHeaders::clear()
 {
-    if (d->headers.isEmpty())
+    if (isEmpty())
         return;
     d.detach();
     d->headers.clear();