Revert "Remove implicitly unwrapped optional"

This reverts commit e5f8778.

Author: johnno1962

Foundation/URLSession/URLSessionTask.swift

@@ -33,7 +33,7 @@ open class URLSessionTask : NSObject, NSCopying {
     internal var suspendCount = 1
     internal var session: URLSessionProtocol! //change to nil when task completes
     internal let body: _Body
-    fileprivate var _protocol: URLProtocol? = nil
+    fileprivate var _protocol: URLProtocol! = nil
     private let syncQ = DispatchQueue(label: "org.swift.URLSessionTask.SyncQ")
 
     /// All operations must run on this queue.
@@ -178,8 +178,8 @@ open class URLSessionTask : NSObject, NSCopying {
             self.workQueue.async {
                 let urlError = URLError(_nsError: NSError(domain: NSURLErrorDomain, code: NSURLErrorCancelled, userInfo: nil))
                 self.error = urlError
-                self._protocol?.stopLoading()
-                self._protocol?.client?.urlProtocol(self._protocol!, didFailWithError: urlError)
+                self._protocol.stopLoading()
+                self._protocol.client?.urlProtocol(self._protocol, didFailWithError: urlError)
             }
         }
     }
@@ -235,7 +235,7 @@ open class URLSessionTask : NSObject, NSCopying {
 
             if self.suspendCount == 1 {
                 self.workQueue.async {
-                    self._protocol?.stopLoading()
+                    self._protocol.stopLoading()
                 }
             }
         }
@@ -250,7 +250,7 @@ open class URLSessionTask : NSObject, NSCopying {
             self.updateTaskState()
             if self.suspendCount == 0 {
                 self.workQueue.async {
-                    self._protocol?.startLoading()
+                    self._protocol.startLoading()
                 }
             }
         }