Make optional the required argument in have_db_index(...).unique

I was caught out when I skipped the argument in `have_db_index(...).unique` because I thought the method declaration may have been similar to `validates_uniqueness_of(...).case_insensitive` (although `case_insensitive` defaults to *true* always.) – so I've made this change.

There are some other places where similar changes could be made, which would further improve consistency, but I'll hold off making further changes until I get your thoughts.