If the server failed on us before we even tried to write any bytes,
it's safe to retry the request on a new connection, regardless of the
HTTP method/idempotence.

Fixes #15723

Change-Id: I25360f82aac530d12d2b3eef02c43ced86e62906
Reviewed-on: https://go-review.googlesource.com/27117Reviewed-by: Andrew Gerrand <adg@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>