net/http: clarify ErrBodyNotAllowed error message
net/http: clarify ErrBodyNotAllowed error message

It's usually due to writing on HEAD requests.

R=golang-dev, rsc, r, r
CC=golang-dev
http://codereview.appspot.com/6206106
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go