title: Changing allowsCellularAccess on existing NSURLSession
body: <p>Is it possible to change the value for <code>allowsCellularAccess</code> on an existing <code>NSURLSession</code> by modifying the underlying <code>NSURLSessionConfiguration</code>?</p>
  
  <p>I want to honor any changes in a user's settings for my application without cancelling existing requests if their device is currently connected to WiFi.</p>