Ensure the transfer state is properly updated with the task's response

We seem to be dropping the response in this case, just ensure we actually save it
after the transfer has completed.

Author: harlanhaskins

Sources/FoundationNetworking/URLSession/NativeProtocol.swift

@@ -224,16 +224,15 @@ internal class _NativeProtocol: URLProtocol, _EasyHandleDelegate {
             failWith(error: error!, request: request)
             return
         }
-        guard case .transferInProgress(let ts) = internalState else {
+        guard case .transferInProgress(var ts) = internalState else {
             fatalError("Transfer completed, but it wasn't in progress.")
         }
         guard let request = task?.currentRequest else {
             fatalError("Transfer completed, but there's no current request.")
         }
 
         if let response = task?.response {
-            var transferState = ts
-            transferState.response = response
+            ts.response = response
         }
 
         guard let response = ts.response else {