title: estimatedAssetCount returns a wrong count
body: <p>I need to display the camera roll album with the count of images in it. I'm using the below code to get the camera roll album.</p>
  
  <pre><code>let smartCollections = PHAssetCollection.fetchAssetCollectionsWithType(.SmartAlbum, subtype: .SmartAlbumUserLibrary, options: nil)
  smartCollections.enumerateObjectsUsingBlock { object, index, stop in
      if let collection = object as? PHAssetCollection {
          print(collection.estimatedAssetCount)
      }
  }
  </code></pre>
  
  <p>I have only 28 images in the camera roll in the Photos app. But the <code>estimatedAssetCount</code> property returns the value 9223372036854775807!</p>
  
  <p>This only happens for OS created albums like the camera roll. For user created regular albums, the correct value is returned. Am I doing anything wrong or is this a bug?</p>
  
  <p>If it is, is there any other way to get the correct image count?</p>