magento/magento2:#19101 - API REST and Reserved Order Id

- Fixed issue "Can not update cart with a reserved order number like 000000651"

Author: Vasilii Burlacu

app/code/Magento/Quote/Api/Data/CartInterface.php

@@ -223,14 +223,14 @@ public function setBillingAddress(\Magento\Quote\Api\Data\AddressInterface $bill
     /**
      * Returns the reserved order ID for the cart.
      *
-     * @return int|null Reserved order ID. Otherwise, null.
+     * @return string|null Reserved order ID. Otherwise, null.
      */
     public function getReservedOrderId();
 
     /**
      * Sets the reserved order ID for the cart.
      *
-     * @param int $reservedOrderId
+     * @param string $reservedOrderId
      * @return $this
      */
     public function setReservedOrderId($reservedOrderId);