title: Apple push notification sent_at in apn_notification table is null
body: <p>I use following code with apn_notfications in Ruby</p>
  
  <pre><code>device = APN::Device.create(:token =&gt; "XXXXXXXX XXXXXXXX XXXXXXXX
  XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX XXXXXXXX")
  notification = APN::Notification.new
  device.id= 'xxxxxxx'
  notification.device = device
  notification.badge = 5
  notification.sound = true
  notification.alert = "foobar"
  notification.save
  </code></pre>
  
  <p>But for some notification in apn_notifications table I got sent_at as nil.</p>
  
  <p>Please tell me why this happen and when sent_at field is updated in Table.</p>
  
  <p>Any help would be highly appreciated.</p>