Add a default 'current-menu-item' class

gistfile1.php

add_filter( 'wp_nav_menu_objects', 'edit_nav_main_objects', 10, 2 );
/**
 * Parse nav menu items and add a current-menu-item class to the first menu item
 * in in the menu if no current-menu-item currently exists.
 *
 * @param $current_menu_items array of objects containing menu items
 * @param $current_menu object containing menu information
 *
 * @return mixed array of objects containing menu items
 */
function edit_nav_main_objects( $current_menu_items, $current_menu ) {

	if ( ! isset( $current_menu->theme_location ) || 'header' != $current_menu->theme_location )
		return $current_menu_items;

	$current_classes = wp_list_pluck( $current_menu_items, 'classes' );
	foreach ( $current_classes as $c ) {
		if ( in_array( 'current-menu-item', $c ) )
			return $current_menu_items;
	}

	$current_menu_items[1]->classes[] = 'current-menu-item';

	return $current_menu_items;

}