Preserve the expire date when order completed

When order complete, the expire date were lost.

in license-manager-for-woocommerce/includes/integrations/woocommerce/Controller.php

line 343 ‘expires_at’ => $expiresAt,

however $expiresAt is initialized as null at line 331

I suggest change to $expiresAt = $license->getExpiresAt();

Changed version:

/**
     * Mark the imported license keys as sold.
     *
     * @param LicenseResourceModel[] $licenses License key resource models
     * @param int                    $orderId  WooCommerce Order ID
     * @param int                    $amount   Amount to be marked as sold
     *
     * @throws Exception
     * @throws Exception
     */
    public function sellImportedLicenseKeys($licenses, $orderId, $amount)
    {
        $cleanLicenseKeys = $licenses;
        $cleanOrderId     = $orderId ? absint($orderId) : null;
        $cleanAmount      = $amount  ? absint($amount)  : null;

        if (!is_array($licenses) || count($licenses) <= 0) {
            throw new Exception('License Keys are invalid.');
        }

        if (!$cleanOrderId) {
            throw new Exception('Order ID is invalid.');
        }

        if (!$cleanOrderId) {
            throw new Exception('Amount is invalid.');
        }

        for ($i = 0; $i < $cleanAmount; $i++) {
            /** @var LicenseResourceModel $license */
            $license   = $cleanLicenseKeys[$i];
            $validFor  = intval($license->getValidFor());
            $expiresAt = $license->getExpiresAt();

            if ($validFor) {
                $date         = new DateTime();
                $dateInterval = new DateInterval('P' . $validFor . 'D');
                $expiresAt    = $date->add($dateInterval)->format('Y-m-d H:i:s');
            }

            LicenseResourceRepository::instance()->update(
                $license->getId(),
                array(
                    'order_id'   => $cleanOrderId,
                    'expires_at' => $expiresAt,
                    'status'     => LicenseStatus::SOLD
                )
            );
        }
    }