Add ability to set apns-push-type header

[josippetric]

In iOS 13 it is required to set apns-push-type to background if we want to use silent push notifications, otherwise notifications won't be delivered.

Is there a way to set that header?

[ricardopereira]

Related with #74.

[ricardopereira]

Possible solutions: see my #74 (comment).

[pakko972]

Hello, when I select `Conserve-power (5)' for the priority, my silent notifications are successfully delivered.