fixed GROUP BY issue with expressions

[dbaldy]

Fixed this issue: #12

It was due to a specificity of MSSQL used through pyodbc that doesn't allow using a column in an expression without referring to it in the GROUP BY clause.

In other terms, this is not allowed with MSSQL's sp_prepexec:
SELECT id, F(a) FROM ... GROUP BY id, F(a)

But should instead be:
SELECT id, F(a) FROM ... GROUP BY id, F(a), a

The PR should be compatible with 2.2, 3.0 and 3.1.

[OskarPersson]

Please rebase this and fix the failing tests