Fix serialization for grouped notification

Author: Holofox

android/src/main/java/com/onesignal/flutter/OneSignalSerializer.java

@@ -135,7 +135,7 @@ static HashMap<String, Object> convertNotificationToMap(OSNotification notificat
             for (OSNotification groupedNotification : notification.getGroupedNotifications())
                 payloadJsonArray.put(groupedNotification.toJSONObject());
 
-            hash.put("groupedNotifications", payloadJsonArray);
+            hash.put("groupedNotifications", payloadJsonArray.toString());
         }
 
         hash.put("notificationId", notification.getNotificationId());

lib/src/notification.dart

@@ -217,9 +217,10 @@ class OSNotification extends JSONStringRepresentable {
           json['backgroundImageLayout'].cast<String, dynamic>());
     }
     if (json.containsKey('groupedNotifications')) {
-      String jsonGroupedNotifications = json['groupedNotifications'] as String;
-      List jsonList = jsonDecode(jsonGroupedNotifications) as List;
-      this.groupedNotifications = jsonList.map((item) => OSNotification(item)).toList();
+      final dynamic jsonGroupedNotifications = json['groupedNotifications'];
+      final jsonList = jsonDecode(jsonGroupedNotifications.toString()) as List<dynamic>;
+      this.groupedNotifications = jsonList.map((dynamic item) =>
+          OSNotification(item as Map<String, dynamic>)).toList();
     }
 
     // shared parameters