code: function login (config) {
    config = Object.assign({}, defaultConfig, config)
    return loginTo(AUTH_ENDPOINTS.PRIMARY, config)
      .then(webId => webId || loginTo(AUTH_ENDPOINTS.SECONDARY, config))
  }
docs: "Logs in"* to a WebID-TLS server by sending a HEAD request.  The TLS
  handshake happens implicitly through either an 'XMLHttpRequest' or node
  'https.request'.
  
  First tries to log in to the "primary" WebID-TLS endpoint, and retries with
  the "fallback" endpoint if the first attempt fails.
  
  @param {Object} config - the configuration object specified in {@link
  module:config-default}.
  @returns {(Promise<String>|Promise<null>)} the WebID as a string if the
  client cert is recognized, otherwise null.