Initializes HTTP request formats.
protected static function initializeFormats() {
  static::$formats = array(
    'html' => array(
      'text/html',
      'application/xhtml+xml',
    ),
    'txt' => array(
      'text/plain',
    ),
    'js' => array(
      'application/javascript',
      'application/x-javascript',
      'text/javascript',
    ),
    'css' => array(
      'text/css',
    ),
    'json' => array(
      'application/json',
      'application/x-json',
    ),
    'xml' => array(
      'text/xml',
      'application/xml',
      'application/x-xml',
    ),
    'rdf' => array(
      'application/rdf+xml',
    ),
    'atom' => array(
      'application/atom+xml',
    ),
    'rss' => array(
      'application/rss+xml',
    ),
  );
}