title: WP Bootstrap Navwalker not displaying 'current-menu-item' class
body: <p>The <code>li</code> that WP Bootstrap Navwalker generates doesn't include "current-menu-item" class causing me problem styling the current active menu item.</p>
  
  <p>I added the code below in my <code>functions.php</code> file but it's not working because the <code>li</code> don't have 'current-menu-item' class to begin with.</p>
  
  <pre><code>function special_nav_class ($classes, $item) {
    if (in_array('current-menu-item', $classes) ){
      $classes[] = 'active ';
    }
    return $classes;
  }
  </code></pre>
  
  <p>I also tried declaring a global variable <code>$myNav</code> and then assigned the whole <code>wp_nav_menu</code> code block but still not working.</p>
  
  <p>Here's how my <code>wp_nav_menu</code> code inside my <code>header.php</code> file look like.</p>
  
  <pre><code>&lt;?php
    wp_nav_menu( array(
      'theme_location'  =&gt;    'primary-menu',
      'depth'           =&gt;    1, // 1 = no dropdowns, 2 = with dropdowns.
      'container'       =&gt;    'div',
      'container_class' =&gt;    'collapse navbar-collapse',
      'container_id'    =&gt;    'nav-bar',
      'menu_class'      =&gt;    'navbar-nav mr-auto',
      'fallback_cb'     =&gt;    'WP_Bootstrap_Navwalker::fallback',
      'walker'          =&gt;    new WP_Bootstrap_Navwalker(),
  ) );
  ?&gt;
  </code></pre>
  
  <p>Here's my <code>functions.php</code> file.</p>
  
  <pre><code>// Register Custom Navigation Walker
  if ( ! file_exists( get_template_directory() . '/class-wp-bootstrap-navwalker.php' ) ) {
      // file does not exist... return an error.
      return new WP_Error( 'class-wp-bootstrap-navwalker-missing', __( 'It appears the class-wp-bootstrap-navwalker.php file may be missing.', 'wp-bootstrap-navwalker' ) );
  } else {
      // file exists... require it.
      require_once get_template_directory() . '/class-wp-bootstrap-navwalker.php';
  }
  
  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;
  }
  
  // Register WordPress nav menu
  register_nav_menu('top', 'Top menu');
  </code></pre>
  
  <p>Here's how it looks like when I inspect element on the output HTML.</p>
  
  <pre><code>&lt;div id="nav-bar" class="collapse navbar-collapse"&gt;
    &lt;ul id="menu-main-menu" class="navbar-nav mr-auto"&gt;
      &lt;li itemscope="itemscope" itemtype="https://www.schema.org/SiteNavigationElement" id="menu-item-11" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-11 nav-item"&gt;
        &lt;a title="Home" href="#home-page" class="nav-link"&gt;Home&lt;/a&gt;
      &lt;/li&gt;
      &lt;li itemscope="itemscope" itemtype="https://www.schema.org/SiteNavigationElement" id="menu-item-12" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-12 nav-item"&gt;
        &lt;a title="Our Company" href="#our-company" class="nav-link"&gt;Our Company&lt;/a&gt;
      &lt;/li&gt;
      &lt;li itemscope="itemscope" itemtype="https://www.schema.org/SiteNavigationElement" id="menu-item-13" class="menu-item menu-item-type-custom menu-item-object-custom menu-item-13 nav-item"&gt;
        &lt;a title="Our Products" href="#our-products" class="nav-link"&gt;Our Products&lt;/a&gt;
      &lt;/li&gt;
    &lt;/ul&gt;
  &lt;/div&gt;
  </code></pre>
  
  <p>My goal is to get the navwalker to generate <code>current-menu-item</code> class in the active <code>li</code> so I can style it properly.</p>