The if_not_exists option is silently ignored

The if_not_exists option is currently broken and silently ignored because ActiverecordCoveringIndex::PostgreSQLAdapter#add_index_options just consumes it without passing it to the super method.

Is this project not accepting merge requests? I created one that fixes this bug in my fork, which you may want to take a look. https://gitlab.com/knu/activerecord-covering-index/-/merge_requests/1