net/http: fix flaky TestTransportCancelBeforeResponseHeaders test

Add a couple more cases where we convert random network I/O errors
into errRequestCanceled if the request was forcefully aborted.

It failed ~1/1000 times without -race, or very easily with -race.
(due to -race randomizing some scheduling)

Fixes #11894

Change-Id: Ib1c123ce1eebdd88642da28a5948ca4f30581907
Reviewed-on: https://go-review.googlesource.com/18287Reviewed-by: Russ Cox <rsc@golang.org>

src/net/http/transport.go

@@ -1423,6 +1423,9 @@ WaitResponse:
 		select {
 		case err := <-writeErrCh:
 			if err != nil {
+				if pc.isCanceled() {
+					err = errRequestCanceled
+				}
 				re = responseAndError{err: beforeRespHeaderError{err}}
 				pc.close(fmt.Errorf("write error: %v", err))
 				break WaitResponse
@@ -1446,6 +1449,9 @@ WaitResponse:
 			re = responseAndError{err: errTimeout}
 			break WaitResponse
 		case re = <-resc:
+			if re.err != nil && pc.isCanceled() {
+				re.err = errRequestCanceled
+			}
 			break WaitResponse
 		case <-cancelChan:
 			pc.t.CancelRequest(req.Request)

src/net/http/transport_test.go

@@ -1649,7 +1649,6 @@ func TestCancelRequestWithChannelBeforeDo(t *testing.T) {
 
 // Issue 11020. The returned error message should be errRequestCanceled
 func TestTransportCancelBeforeResponseHeaders(t *testing.T) {
-	setFlaky(t, 11894)
 	defer afterTest(t)
 
 	serverConnCh := make(chan net.Conn, 1)