Add unit test for List.of().

xstream/src/test/com/thoughtworks/acceptance/Collections09Test.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2021 XStream Committers.
+ * All rights reserved.
+ *
+ * The software in this package is published under the terms of the BSD
+ * style license a copy of which has been included with this distribution in
+ * the LICENSE.txt file.
+ *
+ * Created on 15. May 2021 by Joerg Schaible
+ */
+package com.thoughtworks.acceptance;
+
+import java.util.List;
+
+
+public class Collections09Test extends AbstractAcceptanceTest {
+
+    public void testListFromListOf() {
+        xstream.allowTypes("java.util.CollSer");
+
+        final List<String> list = List.of("hi", "bye");
+
+        assertBothWays(list, ""//
+            + "<java.util.ImmutableCollections_-List12 resolves-to=\"java.util.CollSer\" serialization=\"custom\">\n"
+            + "  <java.util.CollSer>\n"
+            + "    <default>\n"
+            + "      <tag>1</tag>\n"
+            + "    </default>\n"
+            + "    <int>2</int>\n"
+            + "    <string>hi</string>\n"
+            + "    <string>bye</string>\n"
+            + "  </java.util.CollSer>\n"
+            + "</java.util.ImmutableCollections_-List12>");
+    }
+}