net/http: refactor body logic in test
net/http: refactor body logic in test

This just eliminates some duplication.
Also add a pointer to RFC 1122, in case
this comes up again.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6229044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go