*  **Proxy authenticators are now asked for preemptive authentication.** OkHttp will now request
    authentication credentials before creating TLS tunnels through HTTP proxies (HTTP `CONNECT`).
    Authenticators should identify preemptive authentications by the presence of a challenge whose
    scheme is "OkHttp-Preemptive".

      }

    /**
     * Sets the authenticator used to respond to challenges from origin servers. Use
     * [proxyAuthenticator] to set the authenticator for proxy servers.
     *
     * If unset, the [no authentication will be attempted][Authenticator.NONE].
     */
    fun authenticator(authenticator: Authenticator) =
      apply {
        this.authenticator = authenticator
      }

 * no mechanism to differentiate empty from absent. Neither of these components are popular in
 * practice. Typically HTTP applications use other mechanisms for user identification and
 * authentication.
 *
 * ### Host
 *
 * The host identifies the webserver that serves the URL's resource. It is either a hostname like
 * `square.com` or `localhost`, an IPv4 address like `192.168.0.1`, or an IPv6 address like `::1`.