net/http: use r.Body.Close to close connection during TestServeFileFromCWD
net/http: use r.Body.Close to close connection during TestServeFileFromCWD

Fixes issue 3917.

R=golang-dev, bradfitz
CC=golang-dev
http://codereview.appspot.com/6553061

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang