Fix TestURLSession.test_noDoubleCallbackWhenCancellingAndProtocolFailsFast()

- Darwin requires implementation of FailFastProtocol.canonicalRequest()
  to override abstract method in URLProtocol

- Fix the error code from in the callback to be equal to NSURLErrorCancelled.

- This test now passes on Darwin and Linux but is currently left disabled.

Author: spevans

Tests/Foundation/Tests/TestURLSession.swift

@@ -1004,7 +1004,7 @@ class TestURLSession : LoopbackServerTest {
             callbackCount += 1
             XCTAssertNotNil(error)
             if let urlError = error as? URLError {
-                XCTAssertNotEqual(urlError._nsError.code, NSURLErrorCancelled)
+                XCTAssertEqual(urlError._nsError.code, NSURLErrorCancelled)
             }
 
             if callbackCount == 1 {
@@ -1405,6 +1405,10 @@ class FailFastProtocol: URLProtocol {
         return request.url?.scheme == "failfast"
     }
 
+    override class func canonicalRequest(for request: URLRequest) -> URLRequest {
+        return request
+    }
+
     override class func canInit(with task: URLSessionTask) -> Bool {
         guard let request = task.currentRequest else { return false }
         return canInit(with: request)