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

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>