*  Fix: Change the signature of `Dispatcher` to accept a nullable `ExecutorService`. Changing this
    parameter to be non-null was an unintended signature change in OkHttp 4.0.

 *  New: `EventListener.retryDecision()` is called each time a request fails with an `IOException`.
    It notifies your listener if OkHttp will retry.

 *  New: `EventListener.followUpDecision()` is called each time a response is received. It notifies