[FIX] orm.read_group: improve test coverage and corner case handling:

- allow leading spaces in orderby spec for m2o columns
- extra test for read_group on multiple columns
- proper handling of groupby on numeric column with default order
[FIX] orm.read_group: improve test coverage and corner case handling:

- allow leading spaces in orderby spec for m2o columns
- extra test for read_group on multiple columns
- proper handling of groupby on numeric column with default order