Updates for RFC 8617

Now that the IETF published [RFC 8617](https://www.rfc-editor.org/info/rfc8617), we should update the test suite.

In particular, the instance tag for AMS and AS headers now MUST appear before all other tags (e.g. see the ABNF in [§ 4.1.3](https://www.rfc-editor.org/rfc/rfc8617.html#section-4.1.3))