QHeaderView:setDefaultSectionSize add invalidateCachedSizeHint call

We cannot rely on the previous sizeHint and we also call it
in QHeaderView::resizeSection(). There seems to be no reason
not to call it.

Change-Id: Ia473fb7163369aafa5c019ff97a030f9f54a33d9
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

src/widgets/itemviews/qheaderview.cpp

@@ -3370,6 +3370,7 @@ void QHeaderViewPrivate::cascadingResize(int visual, int newSize)
 void QHeaderViewPrivate::setDefaultSectionSize(int size)
 {
     Q_Q(QHeaderView);
+    invalidateCachedSizeHint();
     defaultSectionSize = size;
     for (int i = 0; i < sectionItems.count(); ++i) {
         QHeaderViewPrivate::SectionItem &section = sectionItems[i];