title: urlSession:dataTask:didReceive:completionHandler not called in Xcode8 Swift3
body: <p>I've created a background <code>URLSession</code> object using the following code:</p>
  
  <pre><code>let identifier = /* some background identifier */
  let config = URLSessionConfiguration.background(withIdentifier: identifier)
  self.session = URLSession(configuration: config, delegate: self, delegateQueue: nil)
  </code></pre>
  
  <p>I've also implemented the following <code>URLSessionDataDelegate</code> methods in the delegate class: </p>
  
  <pre><code>public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -&gt; Void) {
      // code goes here
  }
  
  public func urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data) {
      // code goes here
  }
  
  public func urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?) { 
      // code goes here
  }
  </code></pre>
  
  <p>I'm able to successfully call <code>URLSessionUploadTask</code> from this session using the following: </p>
  
  <pre><code>let fileURl = /* some file url */
  let request = /* some URLRequest */
  let task = session.uploadTask(with: request, fromFile: fileUrl)
  task.resume()
  </code></pre>
  
  <p>But for some reason I only get callbacks for: </p>
  
  <pre><code>`urlSession(_ session: URLSession, task: URLSessionTask, didCompleteWithError error: Error?)` 
  </code></pre>
  
  <p>and </p>
  
  <pre><code>`urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive data: Data)`.
  </code></pre>
  
  <p>The delegate method: </p>
  
  <pre><code>`urlSession(_ session: URLSession, dataTask: URLSessionDataTask, didReceive response: URLResponse, completionHandler: @escaping (URLSession.ResponseDisposition) -&gt; Void)`
  </code></pre>
  
  <p>never gets called.</p>
  
  <p>Is this a bug in Xcode8 or am I missing something important?
  Thanks for the help!</p>