Sandbox commands shouldn't try parse all fields as dates

`SandboxCommand` is a subclass of `Message`, so its fields get parsed with [`date_time_decoder`](https://github.com/praekelt/vumi/blob/develop/vumi/message.py#L16-L25). This is causing problems in the sandbox contacts resource for cases where we try save contacts with extra fields that can be parsed with `datetime.strptime`.