We were previously only using the new-ish Request.GetBody to "rewind"
a Request.Body on retry when it seemed that we hadn't started the
read+write body copy process from the old request yet.

Apparently there's a bug somewhere, so this is a safe minimal fix for
now, unconditionally using GetBody when it's available, rather than
only using it when it seems we need to. Should have no performance impact
because it's supposed to be cheap, and this only happens on rare retries
where the server's GOAWAY came in-flight while we were writing a request.

Updates golang/go#25009 (not a fix, but enough for Go 1.11)

Change-Id: Ia462944d4a68cf2fde8d32b7b357b450c509a349
Reviewed-on: https://go-review.googlesource.com/123476Reviewed-by: Ian Lance Taylor <iant@golang.org>