Failure to return to calling function from NSURLSession delegate without killing the task

I'm using Swift in Xcode 6.2 (beta) but had the same problem on the 6.1 release version. I'm trying to use NSURLSession and believe I have it set up correctly (see code below). The problem is that I have a delegate setup to deal with a redirect happening through the code. I actually need to capture the cookies prior to the final redirection and I'm doing this through the delegate:

func URLSession(_:NSURLSession, task:NSURLSessionTask, willPerformHTTPRedirection:NSHTTPURLResponse, newRequest:NSURLRequest, completionHandler:(NSURLRequest!) -> Void )

This works and I'm able to execute code successfully and capture the cookies I need. The problem is that I need to add task.cancel() at the end of the function or else it never seems to complete and return to the delegator (parent?) function. Because of this I lose the results from the redirect URL (although in my current project it is inconsequential). The strange thing is that this was working for a while and seemingly stopped. I don't believe I entered any code that changed it, but something had to happen. Below is the relevant code.

NSURLSession Function:

func callURL (a: String, b: String) -> Void {
    // Define the URL
    var url = NSURL(string: "https://mycorrecturl.com");

    // Define the request object (via string)
    var request = NSMutableURLRequest(URL: url!)

    // Use default configuration
    let config = NSURLSessionConfiguration.defaultSessionConfiguration()

    // Create the NSURLSession object with default configuration, and self as a delegate (so calls delegate method)
    let session = NSURLSession(configuration: config, delegate: self, delegateQueue: nil)

    // Change from default GET to POST (needed to call URL properly)
    request.HTTPMethod = "POST"

    // Construct my parameters to send in with the URL
    var params = ["a":a, "b":b] as Dictionary<String, String>

    var err: NSError?
    request.HTTPBody = NSJSONSerialization.dataWithJSONObject(params, options: nil, error: &err)

    var task = session.dataTaskWithRequest(request, completionHandler: {data, response, error -> Void in
        // Do some other stuff after delegate has returned...
    })
    task.resume()
    return
}

The delegate code:

func URLSession(_:NSURLSession, task:NSURLSessionTask, willPerformHTTPRedirection:NSHTTPURLResponse, newRequest:NSURLRequest, completionHandler:(NSURLRequest!) -> Void ) {
    // Check Cookies
    let url = NSURL(string: "https://mycorrecturl.com")
    var all = NSHTTPCookie.cookiesWithResponseHeaderFields(willPerformHTTPRedirection.allHeaderFields, forURL: url!)

    // Get the correct cookie
    for cookie:NSHTTPCookie in all as [NSHTTPCookie] {
        if cookie.name as String == "important_cookie" {
            NSHTTPCookieStorage.sharedHTTPCookieStorage().setCookie(cookie)
        }
    }
    task.cancel()
}

It used to return to the calling function without calling task.cancel(). Is there anything that looks wrong with the code that would cause it to just hang in the delegate function if task.cancel() isn't called?

Edit: What code would I add to fix this.