add Undefined to EnumWithNullToString4355Test (#4361)

Author: pjfanning

src/test/java/com/fasterxml/jackson/databind/deser/enums/EnumWithNullToString4355Test.java

@@ -33,6 +33,8 @@ public String toString() {
     @Test
     public void testWithNullToString() throws Exception
     {
-        assertEquals(q("ALPHA"), MAPPER.writeValueAsString(Enum4355.ALPHA));        
+        assertEquals(q("ALPHA"), MAPPER.writeValueAsString(Enum4355.ALPHA));
+        assertEquals(q("BETA"), MAPPER.writeValueAsString(Enum4355.BETA));
+        assertEquals(q("UNDEFINED"), MAPPER.writeValueAsString(Enum4355.UNDEFINED));
     }
 }