net/http: fix test to check that requests to 'localhost' are not proxied

I think the test was meant to test requests to 'localhost:80' instead
of 'localhost:80:80'. It passes even with 'localhost:80:80' because
net.SplitHostPort fails inside useProxy. Please comment if you want to
leave old 'localhost:80' is the list too to check old code path.

Change-Id: Ic4cd21901563449e3d4e2f4c8caf723f4ca15bac
u
Reviewed-on: https://go-review.googlesource.com/4293
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/http/proxy_test.go

@@ -18,7 +18,7 @@ var UseProxyTests = []struct {
 	match bool
 }{
 	// Never proxy localhost:
-	{"localhost:80", false},
+	{"localhost", false},
 	{"127.0.0.1", false},
 	{"127.0.0.2", false},
 	{"[::1]", false},