net/http: fix Server blocking after a Handler's Write fails

If a Handle's Write to a ResponseWriter fails (e.g. via a
net.Conn WriteDeadline via WriteTimeout on the Server), the
Server was blocking forever waiting for reads on that
net.Conn, even after a Write failed.

Instead, once we see a Write fail, close the connection,
since it's then dead to us anyway.

Fixes issue 4741

R=golang-dev, adg
CC=golang-dev
https://codereview.appspot.com/7301043