How to add "active" class to wp_nav_menu() current menu item (simple way)

I am creating custom Wordpress theme using a starter theme _Underscores and Bootstrap.

I would like to modify wp_nav_menu so that it assigns the current menu item .active class instead of the default .current-menu-item. I need this in order to use .active class from Bootstrap.

Here is what I have (extra stuff comes from WP so please scroll to the right):

<ul id="menu-main-menu" class="nav navbar-nav">
   <li id="menu-item-14" class="menu-item menu-item-type-post_type menu-item-object-page current-menu-item page_item page-item-13 current_page_item menu-item-14"><a href="">item1</a></li>
   <li id="menu-item-12" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-12"><a href="">item2</a></li>
</ul>

And here is what I need:

<ul id="menu-main-menu" class="nav navbar-nav">
   <li id="menu-item-14" class="active menu-item menu-item-type-post_type menu-item-object-page current-menu-item page_item page-item-13 current_page_item menu-item-14"><a href="">item1</a></li>
   <li id="menu-item-12" class="menu-item menu-item-type-post_type menu-item-object-page menu-item-12"><a href="">item2</a></li>
</ul>

I would prefer to achieve this without modifying ../wp-includes/nav-menu-template.php and without using JS.

UPDATE: I found the answer just before posting this question, but since I had a rather hard time finding it, posting this as a QA to hopefully save someone some time.

Just paste this code into functions.php file:

add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);

function special_nav_class ($classes, $item) {
  if (in_array('current-menu-item', $classes) ){
    $classes[] = 'active ';
  }
  return $classes;
}

More on wordpress.org:

• Highlight Current Page in WordPress 3.0 Menus
• Adding .active class to active menu item

To also highlight the menu item when one of the child pages is active, also check for the other class (current-page-ancestor) like below:

add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);

function special_nav_class ($classes, $item) {
    if (in_array('current-page-ancestor', $classes) || in_array('current-menu-item', $classes) ){
        $classes[] = 'active ';
    }
    return $classes;
}

If you want the 'active' in the html:

header with html and php:

        <?php
        $menu_items = wp_get_nav_menu_items( 'main_nav' ); // id or name of menu
        foreach ( (array) $menu_items as $key => $menu_item ) {
            if ( ! $menu_item->menu_item_parent ) {
                echo "<li class=" . vince_check_active_menu($menu_item) . "><a href='$menu_item->url'>";
                echo $menu_item->title;
                echo "</a></li>";
            }
        }
        ?>

functions.php:

function vince_check_active_menu( $menu_item ) {
    $actual_link = ( isset( $_SERVER['HTTPS'] ) ? "https" : "http" ) . "://$_SERVER[HTTP_HOST]$_SERVER[REQUEST_URI]";
    if ( $actual_link == $menu_item->url ) {
        return 'active';
    }
    return '';
}

In addition to previous answers, if your menu items are Categories and you want to highlight them when navigating through posts, check also for current-post-ancestor:

add_filter('nav_menu_css_class' , 'special_nav_class' , 10 , 2);

function special_nav_class ($classes, $item) {
    if (in_array('current-post-ancestor', $classes) || in_array('current-page-ancestor', $classes) || in_array('current-menu-item', $classes) ){
        $classes[] = 'active ';
    }
    return $classes;
}