Currently the notification, if a sound is specified, will be converted into a critical notification. This behaviour occurs because the sideshow/apns2 package initializes a new sound-dict on `payload.SoundName()` and inserts the name into it. If the sound-key is specified as a dictionary, the alert will be seen as a critical alert according to the documentation [developer.apple.com/documentation/usernotifications](https://developer.apple.com/documentation/usernotifications/setting_up_a_remote_notification_server/generating_a_remote_notification#2943363).
In order to prevent the notification to get a critical one by specifying a sound, the sound name from the rest-API needs to be set as string to the sound-property of the payload for apns. This can be achieved by setting either the sound-key from the request to the sound property of the payload if it is a string or by setting the soundname-key from the request to the sound property without the usage of `payload.SoundName()`. For more consistency to the documentation, this pull request implements the variant, that the sound-key from the request is set as string into the sound-property of the apns-payload.
