Drop cached qualified name after changing nodes in subtree

#KT-2969 Fixed

compiler/frontend/src/org/jetbrains/jet/lang/psi/JetNamespaceHeader.java

@@ -128,5 +128,10 @@ public class JetNamespaceHeader extends JetReferenceExpression {
         }
         return builder.toString();
     }
+
+    @Override
+    public void subtreeChanged() {
+        qualifiedNameCache = null;
+    }
 }