code: function createEnclosure (url, type, length) {
    if (!url) {
      return null;
    }
  
    var enclosure = {url: url};
    if (type) {
      enclosure.type = type;
    }
    if (length !== undefined) {
      enclosure.length = length;
    }
    return enclosure;
  }
docs: Creates an enclosure given a required url and optional type/length.  Although required by the RSS spec, many feeds
  don't include a type or length so we should just rely on url and use that for the id of the enclosures hash.
  
  @param url
  @param type
  @param length
  @returns {{url: *}}