Even existing credit memos should be refundable if their state is open

Credit memos can have the state open: `\Magento\Sales\Model\Order\Creditmemo::STATE_OPEN`.
This means that it is possible to have a creditmemo with an ID which still has to be refunded.
I'm creating a module that introduces a validation step for refund payments before the actual refund can take place. It uses the open state of credit memos to wait for approval and then refunds the creditmemo. Right now the credit memo is not refundable once it has an ID. I think this is incorrect in case of open credit memos.

Author: Anton Evers

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

@@ -195,7 +195,7 @@ public function refund(
      */
     protected function validateForRefund(\Magento\Sales\Api\Data\CreditmemoInterface $creditmemo)
     {
-        if ($creditmemo->getId()) {
+        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.')
             );