Loose integer check for creditmemo state

`$creditmemo->getState() = "1"`
`Creditmemo::STATE_OPEN = 1`

app/code/Magento/Sales/Model/Service/CreditmemoService.php

@@ -195,7 +195,7 @@ class CreditmemoService implements \Magento\Sales\Api\CreditmemoManagementInterf
      */
     protected function validateForRefund(\Magento\Sales\Api\Data\CreditmemoInterface $creditmemo)
     {
-        if ($creditmemo->getId() && $creditmemo->getState() !== \Magento\Sales\Model\Order\Creditmemo::STATE_OPEN) {
+        if ($creditmemo->getId() && $creditmemo->getState() != \Magento\Sales\Model\Order\Creditmemo::STATE_OPEN) {
             throw new \Magento\Framework\Exception\LocalizedException(
                 __('We cannot register an existing credit memo.')
             );