Fixes for breaking changes in iOS 13

[vlasky]

I would expect many of you to already know that Apple has introduced breaking changes to push notifications in iOS 13.

Here is a great article that summarises them:

https://onesignal.com/blog/ios-13-introduces-4-breaking-changes-to-notifications/

I've forked petercv's node-apn-http2 fork to create my own fork that addresses these changes:

• It generates the apns-push-type attribute and automatically sets it to 'background' or 'alert' unless push type is manually set for the notification.
• It automatically sets apns-priority to 5 for pure background notifications unless priority is manually set for the notification.

My fork is work in progress and I may make further changes.