*在第一次调用registerForRemoteNotificationTypes方法时没有联网，则既不会调用didRegisterForRemoteNotificationsWithDeviceToken，也不会调用didFailToRegisterForRemoteNotificationsWithError
*在第一次调用registerForRemoteNotificationTypes注册成功后，之后即使没有联网，再调用registerForRemoteNotificationTypes时都会以最上一次的device token作为参数回调didRegisterForRemoteNotificationsWithDeviceToken方法。
(官方描述) If your application has previously registered, calling registerForRemoteNotificationTypes: results in the operating system passing the device token to the delegate immediately without incurring additional overhead.
