Update Export.php

fix for export grid with the number of lines > 1000 (collection not loaded second time)

Author: NikolasSumrak

app/code/Magento/Backend/Block/Widget/Grid/Export.php

@@ -257,6 +257,7 @@ public function _exportIterateCollection($callback, array $args)
         $break = false;
 
         while ($break !== true) {
+            $originalCollection->clear();
             $originalCollection->setPageSize($this->getExportPageSize());
             $originalCollection->setCurPage($page);
             $originalCollection->load();