Is there a particular reason that varsKey and routeKey are using the contextKey type? https://github.com/gorilla/mux/blob/master/mux.go#L322-L327

This causes problems when attempting to use request.WithContext(context.WithValue(request.Context(), 0, map[string]string{...})) for testing route params because mux.Vars is looking for contextKey(0) as the key rather than int(0).