Setting NSURLSessionConfiguration's HTTPShouldSetCookies to false disables cookie storage - ios

Question

What I want to do is create an NSURLSession that does not automatically give a value to the Cookie HTTP header when I'm making an NSMutableURLRequest, because I set that value on my own. However, I want to be able to store the cookie that comes back to me when I make a sign in API request.
I use the following code to instantiate my session:
func createSession() {
let configuration = NSURLSessionConfiguration.defaultSessionConfiguration()
// Set cookie policies.
configuration.HTTPCookieAcceptPolicy = .Always
configuration.HTTPCookieStorage = NSHTTPCookieStorage.sharedHTTPCookieStorage()
configuration.HTTPShouldSetCookies = false
// ...
return NSURLSession(configuration: configuration, delegate: nil, delegateQueue: nil)
}
When the NSURLSessionDataTask comes back to me in its completionHandler, I dig into the shared cookie storage to get the cookies that came with the response. However, I always get an empty array.
if let cookies = NSHTTPCookieStorage.sharedHTTPCookieStorage().cookiesForURL(response.URL!) {
println("cookies: \(cookies)") // Prints an empty array.
if let cookie = cookies.last as? NSHTTPCookie {
// I save the cookie to disk here.
}
}
Why is this happening even when I've set the NSURLSession to always accept cookies, and even provided a cookie storage?
I can only get the behavior that I want by setting the HTTPShouldSetCookies to true and supplying a Cookie HTTP header in my NSMutableURLRequest anyway. However, the Apple documentation on HTTPShouldSetCookies says that I should set it to false if I were to provide the cookie on my own in the request level:
If you want to provide cookies yourself, set this value to NO and provide a Cookie header either through the session’s HTTPAdditionalHeaders property or on a per-request level using a custom NSURLRequest object.