fix(lib): fix challenge issuance logic (#881)

* fix(lib): fix challenge issuance logic

Fixes #869

v1.21.0 changed the core challenge flow to maintain information about
challenges on the server side instead of only doing them via stateless
idempotent generation functions and relying on details to not change.
There was a subtle bug introduced in this change: if a client has an
unknown challenge ID set in its test cookie, Anubis will clear that
cookie and then throw an HTTP 500 error.

This has been fixed by making Anubis throw a new challenge page instead.

Signed-off-by: Xe Iaso <me@xeiaso.net>

* test(lib): you win this time spell check

Signed-off-by: Xe Iaso <me@xeiaso.net>

---------

Signed-off-by: Xe Iaso <me@xeiaso.net>

lib/anubis.go

@@ -102,6 +102,10 @@ func (s *Server) challengeFor(r *http.Request) (*challenge.Challenge, error) {
 	ckie := ckies[0]
 	chall, err := j.Get(r.Context(), "challenge:"+ckie.Value)
 	if err != nil {
+		if errors.Is(err, store.ErrNotFound) {
+			return s.issueChallenge(r.Context(), r)
+		}
+
 		return nil, err
 	}