在 iOS 上，priority 字段似乎是强制性的.

On iOS the priority field seems mandatory.

{   
  "to": "cHPpZ_s14EA:APA91bG56znW...",
  "priority": "high",
  "notification" : {
    "body" : "hello!",
    "title": "afruz",
    "sound": "default"
  }
}