Sending APNS without sound

If I want to send APNS without sound - it should be without option "sound".
Merge pull request #459 from ShishkinAR/master

[Zend_Mobile] Sending APNS without sound
Revert "Sending APNS without sound"
Merge pull request #460 from zendframework/revert-459-master

Revert "Sending APNS without sound"