This change fills the ContentLength field in the http.Response returned by
ResponseRecorder.Result.

Fixes #16952.

Change-Id: I9c49b1bf83e3719b5275b03a43aff5033156637d
Reviewed-on: https://go-review.googlesource.com/28302Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>