net/http/cgi: fix test case sensitivity on Windows
net/http/cgi: fix test case sensitivity on Windows

Fixes issue 4645

R=golang-dev, alex.brainman, minux.ma
CC=golang-dev
https://codereview.appspot.com/7105047
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go