The doc for ResponseRecorder.Result guarantees that the body of the returned
http.Response will be non-nil, but this only holds true if the caller's body is
non-nil. With this change, if the caller's body is nil then the returned
response's body will be an empty io.ReadCloser.

Fixes #26442

Change-Id: I3b2fe4a2541caf9997dbb8978bbaf1f58cd1f471
GitHub-Last-Rev: d802967d89e89c50077fb2d0d455163dcea0eb43
GitHub-Pull-Request: golang/go#26453
Reviewed-on: https://go-review.googlesource.com/124875Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>