function set(part, value, fn) {
  var url = this;

  switch (part) {
    case 'query':
      if ('string' === typeof value && value.length) {
        value = (fn || qs.parse)(value);
      }

      url[part] = value;
      break;

    case 'port':
      url[part] = value;

      if (!required(value, url.protocol)) {
        url.host = url.hostname;
        url[part] = '';
      } else if (value) {
        url.host = url.hostname +':'+ value;
      }

      break;

    case 'hostname':
      url[part] = value;

      if (url.port) value += ':'+ url.port;
      url.host = value;
      break;

    case 'host':
      url[part] = value;

      if (port.test(value)) {
        value = value.split(':');
        url.port = value.pop();
        url.hostname = value.join(':');
      } else {
        url.hostname = value;
        url.port = '';
      }

      break;

    case 'protocol':
      url.protocol = value.toLowerCase();
      url.slashes = !fn;
      break;

    case 'pathname':
    case 'hash':
      if (value) {
        var char = part === 'pathname' ? '/' : '#';
        url[part] = value.charAt(0) !== char ? char + value : value;
      } else {
        url[part] = value;
      }
      break;

    case 'username':
    case 'password':
      url[part] = encodeURIComponent(value);
      break;

    case 'auth':
      var index = value.indexOf(':');

      if (~index) {
        url.username = value.slice(0, index);
        url.username = encodeURIComponent(decodeURIComponent(url.username));

        url.password = value.slice(index + 1);
        url.password = encodeURIComponent(decodeURIComponent(url.password));
      } else {
        url.username = encodeURIComponent(decodeURIComponent(value));
      }
  }

  for (var i = 0; i < rules.length; i++) {
    var ins = rules[i];

    if (ins[4]) url[ins[1]] = url[ins[1]].toLowerCase();
  }

  url.auth = url.password ? url.username +':'+ url.password : url.username;

  url.origin = url.protocol !== 'file:' && isSpecial(url.protocol) && url.host
    ? url.protocol +'//'+ url.host
    : 'null';

  url.href = url.toString();

  return url;
}
This is convenience method for changing properties in the URL instance to
insure that they all propagate correctly.

@param {String} part          Property we need to adjust.
@param {Mixed} value          The newly assigned value.
@param {Boolean|Function} fn  When setting the query, it will be the function
                              used to parse the query.
                              When setting the protocol, double slash will be
                              removed from the final url if it is true.
@returns {URL} URL instance for chaining.
@public