if xml element is set to the same value, ignore the set, mark not as dirty.

lib/src/kdbx_xml.dart

@@ -72,6 +72,9 @@ abstract class KdbxSubTextNode<T> extends KdbxSubNode<T> {
 
   @override
   void set(T value) {
+    if (get() == value) {
+      return;
+    }
     node.isDirty = true;
     final el =
         node.node.findElements(name).singleWhere((x) => true, orElse: () {