httputils: use pointer to CacheBusters in test to fix govet
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 <395a7d33bec8475c9b83b7d440f141bcbd994aa5@golang.org>

golang/gddo,golang/gddo,segmentio/gddo,segmentio/gddo,segmentio/gddo,segmentio/gddo,golang/gddo,golang/gddo