Add test for merging toString with super toString

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1648925 13f79535-47bb-0310-9956-ffa450edef68

src/test/java/org/apache/commons/lang3/builder/JsonToStringStyleTest.java

@@ -86,6 +86,9 @@ public class JsonToStringStyleTest {
                                         + "]").append("a", "hello").toString());
         assertEquals("{\"a\":\"hello\"}", new ToStringBuilder(base)
                 .appendSuper(null).append("a", "hello").toString());
+
+        assertEquals("{\"a\":\"hello\",\"b\":\"world\"}", new ToStringBuilder(base)
+                .appendSuper("{\"a\":\"hello\"}").append("b", "world").toString());
     }
 
     @Test