Change ExampleResponseRecorder to use httptest.NewRequest instead of
http.NewRequest. This makes the example shorter and shows how to use
one more function from the httptest package.

Change-Id: I3d35869bd0a4daf1c7551b649428bb2f2a45eba2
Reviewed-on: https://go-review.googlesource.com/24480
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>