When trying to create my own session object `NSURLSession()` and request an url I get an unrecognized selector exception but when I use the shared session `NSURLSession.sharedSession()` everything works fine. How come?

```
var url = NSURL(string: "http:/www.google.com")
if url != nil {
    //throws unrecognized selector when dataTaskWithURL is called
    let session=NSURLSession()
    session.dataTaskWithURL(url!)

   //works
    let sharedSession=NSURLSession.sharedSession()
    sharedSession.dataTaskWithURL(url!)
}

```