[6.x] Allow private-encrypted pusher channels

[onurkose]

pusher/pusher-php-server allows private-encrypted channels but broadcaster implementation missing the definitions so it couldn't pass the broadcasting/auth guard.

[GrahamCampbell]

Thanks for this PR, but the implementation doesn't look quite right.

[GrahamCampbell]

I've corrected this. Your implementation incorrectly replaced private-encrypted-private-123 with 123 instead of private-123.

[onurkose]

Great, I was trying to find a better way just like yours.

[onurkose]

This is my first contribution after many years of using the framework, so thank all you guys for such a great work.

[driesvints]

@onurkose thanks! :)