ForceReply doesn't work with the new attach argument

[paolobarbolini]

In commit 533a81c the Buttons and the ButtonsRow class were added without implementing the BaseObject class which already had the serialize method for serializing the data to json, instead a new _serialize_attachment method was used without adding it to the ForceReply class, resulting in the impossibility of using ForceReply in botogram 1.0 when the deprecated extra argument will be removed.