minimalistic silent push notification support

[kjostling]

Made a small change to enable sending push notifications without body and subject (ie silent push notifications). Also forced the content_available parameter, if data is set, but Im not sure if thats needed since I could use "setParameter" to do that.

The "real" implementation should probably be to add a "setSilent" method and adjust the payload accordingly.

Good enough?

[LKaemmerling]

It doesn't make sense to me, to sent a notification to a user without any content... @Lloople what do you think?

[AshPowell]

@LKDevelopment they've already consented to receiving notifications, though this doesn't inform them of anything as its not actually a notification - its more just waking the device so the app can run background updates etc

[Lloople]

It doesn’t make sense to me either. Maybe it’s better to have a method for silent notifications instead of adding code to this one.

[LKaemmerling]

Yeah this would be acceptable for me @kjostling could you please do this on the refactor-to-traits branch (and add tests for this case?)

[kjostling]

I can have a look at that sure.
The use case for silent push notifications is to trigger app functions in background as mentioned by @AshPowell

[kjostling]

Refactored here, hope I got it right.
#67