Fixed filter by label for issues CSV export
CSV export of issues (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1126) limits params with params.permit(IssuableFinder::VALID_PARAMS)
. This doesn't allow scalar/array values, and so does not allow users to filer by label.
This MR fixes that by adding IssuableFinder.valid_params(params)
.