net/http: deflake TestClientRedirect308NoGetBody

In an unrelated CL I found a way to increase the likelihood of latent
flaky tests and found this one.

This is just like yesterday's https://golang.org/cl/37624 and dozens
before it (all remnants from the great net/http test parallelization
of Nov 2016 in https://golang.org/cl/32684).

Change-Id: I3fe61d1645062e5109206ff27d74f573ef6ebb2e
Reviewed-on: https://go-review.googlesource.com/37627
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>

src/net/http/client_test.go

@@ -583,8 +583,9 @@ func TestClientRedirect308NoGetBody(t *testing.T) {
 	if err != nil {
 		t.Fatal(err)
 	}
+	c := &Client{Transport: &Transport{DisableKeepAlives: true}}
 	req.GetBody = nil // so it can't rewind.
-	res, err := DefaultClient.Do(req)
+	res, err := c.Do(req)
 	if err != nil {
 		t.Fatal(err)
 	}