Resolve Can not export Coupon Code to CSV,XML issue29277

Author: edenduong

app/code/Magento/SalesRule/Controller/Adminhtml/Promo/Quote/ExportCouponsXml.php

@@ -15,13 +15,14 @@
 use Magento\Framework\View\Result\Layout;
 use Magento\Framework\App\ResponseInterface;
 use Magento\Framework\App\Action\HttpGetActionInterface;
+use Magento\Framework\App\Action\HttpPostActionInterface;
 
 /**
  * Export coupons to xml file
  *
  * Class \Magento\SalesRule\Controller\Adminhtml\Promo\Quote\ExportCouponsXml
  */
-class ExportCouponsXml extends Quote implements HttpGetActionInterface
+class ExportCouponsXml extends Quote implements HttpGetActionInterface, HttpPostActionInterface
 {
     /**
      * Export coupon codes as excel xml file