auth API: rectify after bumping serial, so the SOA record also gets rectified

[Habbie]

Short description

I think the old behaviour is harmless, as other records (like the apex NSset) will fill out the NSEC3 chain nicely, but it's better to be entirely correct.

Needs a test. (to reproduce, do remember to set SOA-EDIT-API. DEFAULT is good enough.)

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [x] compiled this code
• [x] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master