Make urlSession delegate queue serial as per the spec.

Author: mamabusi

Foundation/NSURLSession/NSURLSession.swift

@@ -211,6 +211,7 @@ open class URLSession : NSObject {
         self.workQueue = DispatchQueue(label: "URLSession<\(identifier)>")
         self.taskAttributesIsolation = DispatchQueue(label: "URLSession<\(identifier)>.taskAttributes", attributes: DispatchQueue.Attributes.concurrent)
         self.delegateQueue = OperationQueue()
+        self.delegateQueue.maxConcurrentOperationCount = 1
         self.delegate = nil
         //TODO: Make sure this one can't be written to?
         // Could create a subclass of URLSessionConfiguration that wraps the
@@ -225,7 +226,9 @@ open class URLSession : NSObject {
         identifier = nextSessionIdentifier()
         self.workQueue = DispatchQueue(label: "URLSession<\(identifier)>")
         self.taskAttributesIsolation = DispatchQueue(label: "URLSession<\(identifier)>.taskAttributes", attributes: DispatchQueue.Attributes.concurrent)
-        self.delegateQueue = queue ?? OperationQueue()
+        let operationQueue = OperationQueue()
+        operationQueue.maxConcurrentOperationCount = 1
+        self.delegateQueue = queue ?? operationQueue
         self.delegate = delegate
         //TODO: Make sure this one can't be written to?
         // Could create a subclass of URLSessionConfiguration that wraps the