Fix #10748: CSV export: add due date column

The CSV export functionality needs to be able to export the due_date
column. However, we don't export the overdue column (yet) because
implementation of this column is a little bit more tricky with the
current CSV API.

core/csv_api.php

@@ -384,3 +384,16 @@ function csv_format_duplicate_id( $p_duplicate_id ) {
 function csv_format_selection( $p_duplicate_id ) {
 	return csv_escape_string( '' );
 }
+
+/**
+ * return the due date column
+ * @param int $p_due_date
+ * @return string
+ * @access public
+ */
+function csv_format_due_date( $p_due_date ) {
+	static $s_date_format = null;
+	if ( $s_date_format === null )
+		$s_date_format = config_get( 'short_date_format' );
+	return csv_escape_string( date( $s_date_format, $p_due_date ) );
+}

core/helper_api.php

@@ -381,6 +381,7 @@ function helper_get_columns_to_view( $p_columns_target = COLUMNS_TARGET_VIEW_PAG
 		$t_keys_to_remove[] = 'selection';
 		$t_keys_to_remove[] = 'edit';
 		$t_keys_to_remove[] = 'bugnotes_count';
+		$t_keys_to_remove[] = 'overdue';
 	}
 
 	$t_enable_sponsorship = config_get( 'enable_sponsorship' );