Fix issue where "base_tax_amount" is inaccurate for Credit Memo items

- Fixes Github issue #2751

Author: Erik Hansen

app/code/Magento/Sales/Model/Order/Creditmemo/Total/Tax.php

@@ -42,7 +42,7 @@ public function collect(\Magento\Sales\Model\Order\Creditmemo $creditmemo)
                  */
 
                 $tax = $orderItemTax - $orderItem->getTaxRefunded();
-                $baseTax = $baseOrderItemTax - $orderItem->getTaxRefunded();
+                $baseTax = $baseOrderItemTax - $orderItem->getBaseTaxRefunded();
                 $discountTaxCompensation = $orderItem->getDiscountTaxCompensationInvoiced() -
                     $orderItem->getDiscountTaxCompensationRefunded();
                 $baseDiscountTaxCompensation = $orderItem->getBaseDiscountTaxCompensationInvoiced() -