fix: enable double slashes for optional path variables

Closes #754

This implementation is flawed, it's making the pass-challenge route
return 404s. I'm not entirely sure why this is happening, but I'm
pushing this for now in case there's some low hanging fruit.

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

lib/challenge/proofofwork/proofofwork_test.go

@@ -9,6 +9,7 @@ import (
 	"github.com/TecharoHQ/anubis/lib/challenge"
 	"github.com/TecharoHQ/anubis/lib/policy"
 	"github.com/TecharoHQ/anubis/lib/policy/config"
+	"github.com/gorilla/mux"
 )
 
 func mkRequest(t *testing.T, values map[string]string) *http.Request {
@@ -124,7 +125,7 @@ func TestBasic(t *testing.T) {
 		t.Run(cs.name, func(t *testing.T) {
 			lg := slog.With()
 
-			i.Setup(http.NewServeMux())
+			i.Setup(mux.NewRouter())
 
 			inp := &challenge.IssueInput{
 				Rule: bot,