fix(lib): fix invalid response after success in Chrome (#711)

Closes #564

This one is really dumb. Take a seat and listen to my tale of woe.

While @victorvalenca was working on #693 we ran into a strange issue.
The tests would consistently pass on Firefox but instantly failed on
Chrome. After adding increasingly desperate debugging logs to the mix,
we found out that somehow Chrome was randomizing the contents of its
Accept-Language header. This was making the challenge string get
calculated differently, thus making things spuriously fail. I cannot
figure out what causes Chrome to do this other than you being in an
environment where you have more than one "system language" set.

Either way, this should finally fix this issue and bring peace to the
land forever*.

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

lib/anubis.go

@@ -77,14 +77,8 @@ type Server struct {
 func (s *Server) challengeFor(r *http.Request, difficulty int) string {
 	fp := sha256.Sum256(s.pub[:])
 
-	acceptLanguage := r.Header.Get("Accept-Language")
-	if len(acceptLanguage) > 5 {
-		acceptLanguage = acceptLanguage[:5]
-	}
-
 	challengeData := fmt.Sprintf(
-		"Accept-Language=%s,X-Real-IP=%s,User-Agent=%s,WeekTime=%s,Fingerprint=%x,Difficulty=%d",
-		acceptLanguage,
+		"X-Real-IP=%s,User-Agent=%s,WeekTime=%s,Fingerprint=%x,Difficulty=%d",
 		r.Header.Get("X-Real-Ip"),
 		r.UserAgent(),
 		time.Now().UTC().Round(24*7*time.Hour).Format(time.RFC3339),