fix writing of string values, also do not trigger dirty event, if entry is already dirty.

lib/src/kdbx_entry.dart

@@ -84,7 +84,8 @@ class KdbxEntry extends KdbxObject {
       }
       return XmlElement(XmlName(KdbxXml.NODE_STRING))
         ..children.addAll([
-          XmlElement(XmlName(KdbxXml.ATTR_PROTECTED))
+          XmlElement(XmlName(KdbxXml.ATTR_PROTECTED)),
+          XmlElement(XmlName(KdbxXml.NODE_KEY))
             ..children.add(XmlText(stringEntry.key.key)),
           value,
         ]);

lib/src/kdbx_object.dart

@@ -68,12 +68,14 @@ abstract class KdbxObject extends KdbxNode {
 
   @override
   set isDirty(bool dirty) {
-    super.isDirty = dirty;
     if (dirty) {
       times.modifiedNow();
+      if (!isDirty && dirty) {
         file.dirtyObject(this);
       }
     }
+    super.isDirty = dirty;
+  }
 
   @override
   XmlElement toXml() {