Update Zone09

[tgreenx]

Purpose

This PR proposes a fix to a bug in Zone09. It also adds a missing return value in a message tag.

Context

Fixes #1210

Relates to zonemaster/zonemaster#1128

Changes

• Dereferencing of array
• Add "rcode" value to message tag Z09_UNEXPECTED_RCODE_MX

How to test this PR

$ zonemaster-cli --test zone/zone09 smart.ondmarc.com --raw
   0.60 WARNING   Z09_UNEXPECTED_RCODE_MX   ns_ip_list=35.246.222.179; rcode=NOTIMPL

$ zonemaster-cli --test zone/zone09 smart.ondmarc.com
Seconds Level     Message
======= ========= =======
   0.62 WARNING   Unexpected RCODE value (NOTIMPL) on the MX query from name servers "35.246.222.179".

[tgreenx]

@matsduf I noticed "rcode" value was passed to the message tag but not used, this is due to an oversight in the specification, but I took the liberty of adding it nonetheless since it is still given in the list of arguments. See https://github.com/zonemaster/zonemaster/blob/v2022.2.2/docs/specifications/tests/Zone-TP/zone09.md?plain=1#L115

EDIT: nevermind, there is already a PR to update the specification. zonemaster/zonemaster#1128

[matsduf]

Our current unit tests do not catch this type of errors. I do not say that we should add such tests to the unit tests, but we should be aware of it.