MAGETWO-31966: Empty title for success placed order page

Author: Vladyslav Shcherbyna

app/code/Magento/Checkout/Controller/Onepage/Success.php

@@ -23,6 +23,7 @@ public function execute()
         //@todo: Refactor it to match CQRS
         $this->_view->loadLayout();
         $this->_view->getLayout()->initMessages();
+        $this->_view->getPage()->getConfig()->getTitle()->set(__('Thank you for your purchase!'));
         $this->_eventManager->dispatch(
             'checkout_onepage_controller_success_action',
             ['order_ids' => [$session->getLastOrderId()]]