fix grouping on expressions (Fixes #515)

- remove spurious rewriting of the source in groupBy
- simplify expr3 with at least one literal argument
- pending test for a trickier case
respond to PR feedback
- add planner test for the specific query mentioned in #515
- remove irrelevant order by in regresssion test
Merge pull request #522 from mossprescott/ready/515

fix grouping with expression in key(s)