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

tv42/old-go,apparentlymart/golang,tv42/old-go,pombredanne/go-deleteme,dsriram/golang,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,dsriram/golang,pombredanne/go-deleteme,gnanderson/go,apparentlymart/golang,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,tv42/old-go,dsriram/golang,tv42/old-go,dsriram/golang,apparentlymart/golang,gnanderson/go,gnanderson/go,dsriram/golang,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go