Fix bug that skips notification on error status 10

The current behavior incorrectly skips a notification when an error
status code of 10 is received from Apple. Consider a scenario where the
notifications array looks like:

notifications = [notification_a, notification_b, notification_c]

Let's say that after writing notification_a, Apple responds with an
error-response packet that has a status code of 10 and an Identifier of
0, indicating that notification_a was the last (and only) notification
successfully sent.

With the current logic, index would be set to 0 and then bumped to 1.
The slice! on notifications would then modify notifications so that its
value is just [notification_c]. So notification_a would've been
successfully sent, notification_c would be queued up for sending, but
notification_b would be dropped and never sent.

Getting rid of the increment to index fixes this bug, so that both
notification_b and notification_c will be queued up for resending.
Merge pull request #44 from rroblak/fix_error_status_10

Fix bug that skips notification on error status 10