仿照mongo，json序列化加上一个_t字段，方便看到类型

Unity/Assets/ThirdParty/LitJson/JsonMapper.cs

@@ -977,6 +977,8 @@ namespace LitJson
             IList<PropertyMetadata> props = type_properties[obj_type];
 
             writer.WriteObjectStart();
+            writer.WritePropertyName("_t");
+            writer.Write(obj_type.Name);
             foreach (PropertyMetadata p_data in props)
             {
                 if (p_data.IsField)