[wordpress] 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. I am also using Bootstrap as a front-end framework.

I would like to modify wp_nav_menu so that it assigns current menu item class="active" instead of class="current-menu-item" (which is default). Or maybe at least assigns both of these classes. I need this in order to use .active class from bootstrap.css.

Here is the example of what I have (all these classes are wp generated, please scroll to see what I mean):

<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 do this in a correct way - don't want to change anything in ..wp-includes/nav-menu-template.php for sure, don't want to use js either.

Well I found the answer just before posting this question (it was all ready, that's why it is still formed in a way as if I am still seeking the answer), but I had rather hard time finding it so I decided to post it as a QA. I hope someone will find this useful.

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;
}

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 '';
}

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;
}

In header.php insert this code to show menu:

<?php
    wp_nav_menu(
        array(
            'theme_location' => 'menu-one',
            'walker' => new Custom_Walker_Nav_Menu_Top
        )
    );
?>

In functions.php use this:

class Custom_Walker_Nav_Menu_top extends Walker_Nav_Menu
{
    function start_el( &$output, $item, $depth = 0, $args = array(), $id = 0 ) {
        $is_current_item = '';
        if(array_search('current-menu-item', $item->classes) != 0)
        {
            $is_current_item = ' class="active"';
        }
        echo '<li'.$is_current_item.'><a href="'.$item->url.'">'.$item->title;
    }

    function end_el( &$output, $item, $depth = 0, $args = array() ) {
        echo '</a></li>';
    }
}