Call base class assignment operator in QHttpNetworkRequest::operator=()

Address a "Do call the base class copy constructor or assignment
operator, if you implement a copy constructor or assignment operator
in a derived class." warning found by an Axivion scan.

Call the assignment operator of the base class explicitly in the
assignment operator of the derived class.

Task-number: QTBUG-125026
Pick-to: 6.7 6.5
Change-Id: I5a76c9ea96275d569b3a036b1e1b791c57d0042f
Reviewed-by: Marc Mutz <marc.mutz@qt.io>
(cherry picked from commit 84bb06fc5699c88b6edc175cc3cf67d88307481b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/network/access/qhttpnetworkrequest.cpp

@@ -262,6 +262,7 @@ void QHttpNetworkRequest::clearHeaders()
 
 QHttpNetworkRequest &QHttpNetworkRequest::operator=(const QHttpNetworkRequest &other)
 {
+    QHttpNetworkHeader::operator=(other);
     d = other.d;
     return *this;
 }