net/http: fix timer leak in Transport when using ResponseHeaderTimeout

Fixes #9104

Change-Id: Ide5ac70215d73278e6910f5b9c7687ad7734c0be
Reviewed-on: https://go-review.googlesource.com/1530Reviewed-by: David Symonds <dsymonds@golang.org>

src/net/http/transport.go

@@ -1087,7 +1087,9 @@ WaitResponse:
 				break WaitResponse
 			}
 			if d := pc.t.ResponseHeaderTimeout; d > 0 {
-				respHeaderTimer = time.After(d)
+				timer := time.NewTimer(d)
+				defer timer.Stop() // prevent leaks
+				respHeaderTimer = timer.C
 			}
 		case <-pconnDeadCh:
 			// The persist connection is dead. This shouldn't