net/http: fix race on bodyEOFSignal.isClosed
net/http: fix race on bodyEOFSignal.isClosed

Update issue 4191.

Fixes unreported race failure at
http://build.golang.org/log/61e43a328fb220801d3d5c88cd91916cfc5dc43c

R=dvyukov, bradfitz
CC=golang-dev
http://codereview.appspot.com/6640057

wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang