net/http/cgi: make test more robust for Windows perl

Update #4401

R=golang-dev, mattn.jp
CC=golang-dev
https://golang.org/cl/6853067

src/pkg/net/http/cgi/testdata/test.cgi

@@ -8,6 +8,8 @@
 use strict;
 use Cwd;
 
+binmode STDOUT;
+
 my $q = MiniCGI->new;
 my $params = $q->Vars;