net/http: make DefaultTransport's Dialer enable DualStack ("Happy Eyeballs")

As @pmarks-net said in the bug, this is something of a prisoner's
dilemma, but it does help people who occasionally report problems.

This is temporary. IPv6 is happening regardless of our decision here,
so we'll do this for now.

Fixes #15324

Change-Id: I8cc29c6efa56222970996c71182fc9ee89d78539
Reviewed-on: https://go-review.googlesource.com/28077
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Gerrand <adg@golang.org>

src/net/http/transport.go

@@ -40,6 +40,7 @@ var DefaultTransport RoundTripper = &Transport{
 	DialContext: (&net.Dialer{
 		Timeout:   30 * time.Second,
 		KeepAlive: 30 * time.Second,
+		DualStack: true,
 	}).DialContext,
 	MaxIdleConns:          100,
 	IdleConnTimeout:       90 * time.Second,