[release-branch.go1.14] net/http: make Transport.RoundTrip preserve Requests

Ensure that the exact Request passed to Transport.RoundTrip
is returned in the Response. Do not replace the Request with
a copy when resetting the request body.

Updates #39533.
Fixes #40973.

Change-Id: Ie6fb080c24b0f6625b0761b7aa542af3d2411817
Reviewed-on: https://go-review.googlesource.com/c/go/+/237560


Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Russ Cox <rsc@golang.org>
Reviewed-on: https://go-review.googlesource.com/c/go/+/249880


Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>