net/http/httptest: updated example to use Result()

net/http/httptest: updated example to use Result()

example for httptest.Recorder was inspecting Recoder directly.
Using Result() to convert Recorder into a http.Response yields a much
better user experience.

Closes #16837

Change-Id: Id0e636c12cd6adb1ba11f89953ff2b0f43758cf3
Reviewed-on: https://go-review.googlesource.com/27495
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>