net/http/httptest: show usage of httptest.NewRequest in example

net/http/httptest: show usage of httptest.NewRequest in example

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>