Fix fatal error in give-export-donations-exporter.php (#7417)

includes/admin/tools/export/give-export-donations-exporter.php

@@ -646,6 +646,7 @@ public function get_percentage_complete() {
 	 * @access public
 	 *
 	 * @since  2.1
+	 * @unreleased Use maybe_serialize() for column data (some data are arrays)
 	 *
 	 * @return string|false
 	 */
@@ -663,6 +664,7 @@ public function print_csv_rows() {
 				foreach ( $row as $col_id => $column ) {
 					// Make sure the column is valid
 					if ( array_key_exists( $col_id, $cols ) ) {
+                        $column = maybe_serialize( $column );
 						$row_data .= '"' . $this->escape_csv_cell_data(preg_replace( '/"/', "'", $column )) . '"';
 						$row_data .= $i == count( $cols ) ? '' : ',';
 						$i ++;