(fix) #108: Missing _sent_notifications

[matiboy]

Define self._sent_notifications even when not using enhanced. Problem is that _sent_notifications is used in send_notification_multiple since f33faf79de5e36a7b983b0a318bd2a7e7f99e3a7 Fixes #108

[jimhorng]

@matiboy , if not using enhanced, it's meaningless to store sent_notifications since APNS will not respond error on fail-to-sent notification. So I suggest better to remove all enhanced-related logic(e.g. sent_notifications) from non-enhanced code. FYR.

[matiboy]

@jimhorng Ok makes sense. So now it's checking whether we're using enhanced before adding the results to the queue (which was causing the original error). Thanks

[jimhorng]

@djacobs , I think this one is good enough to be merged :+1:

[djacobs]

Thanks @jimhorng - will review shortly!