import 421 (Misdirected Request) Status Code from h2

[royfielding]

I think we need to copy the 421 status code definition from Section 9.1.2 of RFC7540 since it is generally applicable for rejecting a misdirected request.

[royfielding]

curious if @martinthomson has an opinion on moving 421 to http-core

[martinthomson]

A slight preference for moving it, I think.

[mnot]

Since we're doing h2bis and since it is applicable more broadly, I agree.

[reschke]

When doing so, we need to consider https://www.rfc-editor.org/errata/eid4666

[royfielding]

Hmm, the errata is about the cacheable paragraph (which is now heuristically) and I was going to remove that for the same reason but decided to import without changes first. I am just going to remove it now.

[reschke]

That might work as well; but then we'll have to mention in "changes" that the default changed.