Add parameter collapse_id

This is APNS header `apns-collapse-id` used to group
notifications. Read more here:
https://developer.apple.com/library/content/documentation/NetworkingInternet/Conceptual/RemoteNotificationsPG/CommunicatingwithAPNs.html#//apple_ref/doc/uid/TP40008194-CH11-SW13

apns2/client.py

@@ -44,14 +44,14 @@ class APNsClient(object):
         self._connection = HTTP20Connection(server, port, ssl_context=ssl_context, force_proto=proto or 'h2')
 
     def send_notification(self, token_hex, notification, topic, priority=NotificationPriority.Immediate,
-                          expiration=None):
+                          expiration=None, collapse_id=None):
-        stream_id = self.send_notification_async(token_hex, notification, topic, priority, expiration)
+        stream_id = self.send_notification_async(token_hex, notification, topic, priority, expiration, collapse_id)
         result = self.get_notification_result(stream_id)
         if result != 'Success':
             raise exception_class_for_reason(result)
 
     def send_notification_async(self, token_hex, notification, topic, priority=NotificationPriority.Immediate,
-                                expiration=None):
+                                expiration=None, collapse_id=None):
         json_str = json.dumps(notification.dict(), cls=self.__json_encoder, ensure_ascii=False, separators=(',', ':'))
         json_payload = json_str.encode('utf-8')
 
@@ -62,6 +62,9 @@ class APNsClient(object):
         if expiration is not None:
             headers['apns-expiration'] = '%d' % expiration
 
+        if collapse_id is not None:
+            headers['apns-collapse-id'] = collapse_id
+
         url = '/3/device/{}'.format(token_hex)
         stream_id = self._connection.request('POST', url, json_payload, headers)
         return stream_id
@@ -75,7 +78,7 @@ class APNsClient(object):
                 data = json.loads(raw_data)
                 return data['reason']
 
-    def send_notification_batch(self, notifications, topic, priority=NotificationPriority.Immediate, expiration=None):
+    def send_notification_batch(self, notifications, topic, priority=NotificationPriority.Immediate, expiration=None, collapse_id=None):
         '''
         Send a notification to a list of tokens in batch. Instead of sending a synchronous request
         for each token, send multiple requests concurrently. This is done on the same connection,
@@ -107,7 +110,7 @@ class APNsClient(object):
             if self.should_send_notification(next_notification, open_streams):
                 logger.info('Sending to token %s', next_notification.token)
                 stream_id = self.send_notification_async(next_notification.token, next_notification.payload, topic,
-                                                         priority, expiration)
+                                                         priority, expiration, collapse_id)
                 open_streams.append(RequestStream(stream_id, next_notification.token))
 
                 next_notification = next(notification_iterator, None)