doh2: respond to invalid requests with proper status code
As of version 5.2.0, DoH(2) only sends 200 OK HTTP status code, or closes the stream inside a connection without any status code. It would be cleaner to reply to at least some invalid requests, see test coverage
tests/config/doh2.test.lua for commented out test cases.
Ignored queries (FORMERR) could also be handled, but it's outside the scope of this ticket, see #471