http: clarify use of w.conn.body in Write when sniffing.
http: clarify use of w.conn.body in Write when sniffing.

R=gri, r, r, rsc
CC=golang-dev
http://codereview.appspot.com/4794047
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go