title: Dashicons not showing in Wordpress 4.9.1?
body: <p>So I'm attempting a little customisation of the admin dashboard in Wordpress (version 4.9.1 bleeding edge). I'm creating a custom post within a plugin that I will be developing for bidding and displaying ad listing posts. The register_post_type function setup is fairly standard and as follows (stripping away anything surplus to requirements):</p>
  
  <pre><code>function tp_register_advert_post_type() {
  
    $singular = 'Ad Listing';
    $plural = 'Ad Listings';
  
    $labels = array(
      'name' =&gt; $plural,
      'singular_name'       =&gt; $singular,
      'add_name'            =&gt; 'Add New',
      'add_new_item'        =&gt; 'Add New ' . $singular,
      'edit'                =&gt; 'Edit',
      'edit_item'           =&gt; 'Edit ' . $singular,
      'new_item'            =&gt; 'New ' . $singular,
      'view'                =&gt; 'View',
      'view_item'           =&gt; 'View ' . $singular,
      'search_item'         =&gt; 'Search ' . $plural,
      'parent'              =&gt; 'Parent ' . $singular,
      'not_found'           =&gt; 'No ' . $plural .' found',
      'not_found_in_trash'  =&gt; 'No ' . $plural .' in Trash'
    );
  
    $args = array(
      'labels'              =&gt; $labels,
      'capability_type'     =&gt; 'post',
      'public'              =&gt; true,
      'publicly_queryable'  =&gt; true,
      'has_archive'         =&gt; true,
      'exclude_from_search' =&gt; false,
      'show_ui'             =&gt; true,
      'show_in_menu'        =&gt; true,
      'show_in_admin_bar'   =&gt; true,
      'menu_position'       =&gt; 6,
      'menu-icon'           =&gt; 'dashicons-vault',
      'can_export'          =&gt; true,
    );
  
    register_post_type( 'ad listing', $args );
  }
  add_action( 'init', 'tp_register_advert_post_type' );
  </code></pre>
  
  <p>Everything works except the menu_icon in the $args array? What exactly have I done wrong here? I can't see anything obvious - everything else works, other than the default "pin" icon is still showing? Help?</p>