net/http: stop timeout timer if user cancels a request

Change-Id: I84faeae69f294b9a70e545faac6a070feba67770
Reviewed-on: https://go-review.googlesource.com/28074
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/client.go

@@ -329,6 +329,7 @@ func setRequestCancel(req *Request, rt RoundTripper, deadline time.Time) (stopTi
 		select {
 		case <-initialReqCancel:
 			doCancel()
+			timer.Stop()
 		case <-timer.C:
 			doCancel()
 		case <-stopTimerCh: