title: KVO in swift array not work use Observable
body: <p>I use Alamofire to request the data from server like code below,and I want to observe the dataSource,I use <a href="https://github.com/slazyk/Observable-Swift" rel="nofollow">https://github.com/slazyk/Observable-Swift</a>.
  but <code>self.tableView?.reloadData()</code> never be called.</p>
  
  <p>ViewModel.swift</p>
  
  <pre><code>var dataSource: [WishOrder] = [WishOrder]()
  var observableDataSource: Observable&lt;[WishOrder]&gt; = Observable([WishOrder]())
      Alamofire.request(Router.WishOrder(type: self.type, page: 1, pageAmount: self.pageAmount))
          .validate()
          .responseCollection { (response:Response&lt;[WishOrder], NSError&gt;) -&gt; Void in
              let result = response.result
              switch result {
              case .Success(let value):
                  self.dataSource = value as [WishOrder]
                  self.observableDataSource.value = self.dataSource
              case .Failure(let error):
                  SVProgressHUD.showErrorWithStatus(error.localizedDescription)
              }
      }
  </code></pre>
  
  <p>ViewController.swift</p>
  
  <pre><code>override func viewDidLoad() {
      super.viewDidLoad()
  
      ...
  
      self.viewModel.observableDataSource.afterChange.add { (_) -&gt; () in
          print("afterChange")
          self.tableView?.reloadData()
      }
  }
  </code></pre>