Support for rich push notification

[sahithchandan]

Hello,

I'm trying to implement rich push notification. I tried to override the format_payload method and added 'mutable-content': True to the aps dictionary. The data that is being posted to the self._make_request('Publish', params, '/', 'POST') has the mutable-content in it but for some reason the notification that come through the device seems to be missing the parameter. It worked initially for few days before it stopped. Can you please look into this?