Added adjustment feature of subscription

This function:
 public function adjust_subscription($subscription_id, $data) {
        $data = array(
            'adjustment' => $data
        );
            
        $result = $this->query('/subscriptions/' . $subscription_id . '/adjustments.json', 'post', $data);

        if ($result->code == 200) {
            $subscription = json_decode($result->response);

            if (count($subscription) == 1) {
                return $subscription->subscription;
            }

            return false;
        }
//        return $result->response;

        $this->error($result->response, $result->code); 
    }