*  Fix: Don't permanently cache responses with `Cache-Control: immutable`. We misunderstood the
    original `immutable` proposal!
 *  Fix: Change `Authenticator`'s `Route` parameter to be nullable. This was marked as non-null but
    could be called with null in some cases.
 *  Fix: Don't create malformed URLs when `MockWebServer` is reached via an IPv6 address.
 *  Fix: Don't crash if the system default authenticator is null.