FIX Use DataColumns content when available in CSV export

Fix #9248 broke #9173 by accidentally checking for a string key in a
numerically indexed array. Change the check to search the array instead
(fixes #10308)

src/Forms/GridField/GridFieldExportButton.php

@@ -234,7 +234,7 @@ class GridFieldExportButton implements GridField_HTMLProvider, GridField_ActionP
                         }
 
                         $value = $columnHeader($relObj);
-                    } elseif ($gridFieldColumnsComponent && array_key_exists($columnSource, $columnsHandled)) {
+                    } elseif ($gridFieldColumnsComponent && in_array($columnSource, $columnsHandled)) {
                         $value = strip_tags(
                             $gridFieldColumnsComponent->getColumnContent($gridField, $item, $columnSource)
                         );