- Fix `group by` key building: populate array indices from
`keyParts` instead of only from `keyRecord`, which was empty for
some expressions (e.g. `tags[1]`).
- Fix collection handling: treat record-like `LuaTables` (no array part)
as items instead of extracting zero items.
- Update test suite.
Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>