make PropertyHelper a bit more backwards compatible

Bugzilla Report 65799

WHATSNEW

@@ -10,6 +10,14 @@ Changes that could break older environments:
   credentials being used on the redirected URI may break.
   Github Pull Request #173
 
+Fixed bugs:
+-----------
+
+* the ProperyEnumerator change introduced in 1.10.9 proved to be not
+  fully backwards compatible when combined with certain custom
+  PropertyHelper implementations - for example when using AntXtras.
+  Bugzilla Report 65799
+
 Other changes:
 --------------
 

src/main/org/apache/tools/ant/PropertyHelper.java

@@ -860,7 +860,7 @@ public class PropertyHelper implements GetProperty {
      * @return the names of all known properties.
      */
     public Set<String> getPropertyNames() {
-        final Set<String> names = new HashSet<>(properties.keySet());
+        final Set<String> names = new HashSet<>(getProperties().keySet());
         getDelegates(PropertyEnumerator.class)
             .forEach(e -> names.addAll(e.getPropertyNames()));
         return Collections.unmodifiableSet(names);