*  The challenge's scheme and realm are now non-null. If you are calling
    `new Challenge(scheme, realm)` you must provide non-null values. These were
    never null in challenges created by OkHttp, but could have been null in
    application code that creates challenges.

 *  New: The `TlsVersion` of a `Handshake` is now non-null. If you are calling
    `Handshake.get()` with a null TLS version, you must instead now provide a

 *     .fragment("_Who?_")
 *     .build();
 * System.out.println(url);
 * ```
 *
 * This prints:
 *
 * ```
 * http://who-let-the-dogs.out/_Who%3F_?_Who?_#_Who?_
 * ```
 *
 * When parsing URLs that lack percent encoding where it is required, this class will percent encode
 * the offending characters.
 *
 * ### IDNA Mapping and Punycode encoding
 *

     *
     * * **Unreachable proxy servers.** A [ProxySelector] can be used to
     *   attempt multiple proxy servers in sequence, eventually falling back to a direct
     *   connection.
     *
     * Set this to false to avoid retrying requests when doing so is destructive. In this case the
     * calling application should do its own recovery of connectivity failures.
     */
    fun retryOnConnectionFailure(retryOnConnectionFailure: Boolean) =