the HTTP response body itself is empty.
 *  Fix: Don't crash when a fast fallback call has both a deferred connection and a held connection.
 *  Fix: `OkHttpClient` no longer implements `Cloneable`. It never should have; the class is
    immutable. This is left over from OkHttp 2.x (!) when that class was mutable. We're using the
    5.x upgrade as an opportunity to remove very obsolete APIs.