add apns-unique-id (#29)

client.go

@@ -185,6 +185,7 @@ func (c *Client) PushWithContext(ctx context.Context, n *Notification) (*Respons
 	r := &Response{}
 	r.StatusCode = response.StatusCode
 	r.ApnsID = response.Header.Get("apns-id")
+	r.ApnsUniqueID = response.Header.Get("apns-unique-id")
 
 	decoder := json.NewDecoder(response.Body)
 	if err := decoder.Decode(r); err != nil && err != io.EOF {

response.go

@@ -135,6 +135,12 @@ type Response struct {
 	// If the value of StatusCode is 410, this is the last time at which APNs
 	// confirmed that the device token was no longer valid for the topic.
 	Timestamp Time
+
+	// An identifier that is only available in the Developement enviroment.
+	// Use this to query Delivery Log information for the corresponding
+	// notification in Push Notifications Console. For more information, see
+	// [Testing notifications using the Push Notification Console](https://developer.apple.com/documentation/usernotifications/testing_notifications_using_the_push_notification_console).
+	ApnsUniqueID string
 }
 
 // Sent returns whether or not the notification was successfully sent.