Fix KDescendantsProxyModel::setSourceModel() not clearing internal caches

This fixes Sune's unittest.

autotests/kdescendantsproxymodeltest.cpp

@@ -77,7 +77,6 @@ void tst_KDescendantProxyModel::testResetModelContent()
                               << "SecondModel1-1";
         QCOMPARE(proxy.rowCount(), results.count());
         for (int i  = 0 ; i < proxy.rowCount() ; i++) {
-            QEXPECT_FAIL("", "KDescendantProxyModel broken in setSourceModel", Continue);
             QCOMPARE(proxy.index(i, 0).data(Qt::DisplayRole).toString(), results[i]);
         }
     }

src/kdescendantsproxymodel.cpp

@@ -246,6 +246,8 @@ QString KDescendantsProxyModel::ancestorSeparator() const
 
 void KDescendantsProxyModel::setSourceModel(QAbstractItemModel *_sourceModel)
 {
+    Q_D(KDescendantsProxyModel);
+
     beginResetModel();
 
     static const char *const modelSignals[] = {
@@ -291,6 +293,11 @@ void KDescendantsProxyModel::setSourceModel(QAbstractItemModel *_sourceModel)
         }
     }
 
+    resetInternalData();
+    if (_sourceModel && _sourceModel->hasChildren()) {
+        d->synchronousMappingRefresh();
+    }
+
     endResetModel();
 }