*  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

     *
     * * **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) =