net/http: add a Linux-only sendfile test
net/http: add a Linux-only sendfile test

I remembered that sendfile support was lacking a test.

R=golang-dev, rsc
CC=golang-dev
http://codereview.appspot.com/5652079
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go