httputils: use pointer to CacheBusters in test to fix govet

Prevent 'govet' from complaining about copying
a lock value ("httputil.CacheBusters contains sync.Mutex").

Change-Id: If2d235f148e767ad0a9e6110ae1f38e69a32b1a9
Reviewed-on: https://go-review.googlesource.com/24260
Reviewed-by: Andrew Gerrand <adg@golang.org>
diff --git a/httputil/buster_test.go b/httputil/buster_test.go
index 0d2bb83..eb44cf4 100644
--- a/httputil/buster_test.go
+++ b/httputil/buster_test.go
@@ -12,7 +12,7 @@
 )
 
 func TestCacheBusters(t *testing.T) {
-	cbs := CacheBusters{Handler: http.FileServer(http.Dir("."))}
+	cbs := &CacheBusters{Handler: http.FileServer(http.Dir("."))}
 
 	token := cbs.Get("/buster_test.go")
 	if token == "" {
@@ -20,7 +20,7 @@
 	}
 
 	var ss StaticServer
-	cbs = CacheBusters{Handler: ss.FileHandler("buster_test.go")}
+	cbs = &CacheBusters{Handler: ss.FileHandler("buster_test.go")}
 
 	token = cbs.Get("/xxx")
 	if token == "" {