Update SqlWalker.php fixed wrong GROUP BY clause on SQL Server platform

Without this patch a query would like like:
```
SELECT c0_.Country AS sclr0
FROM Continent c0_ WITH (NOLOCK)
WHERE c0_.Country = 38
GROUP BY sclr0
```
Using the column alias in the GROUP BY clause. However this is not allowed on SQL Server. References:
1. http://stackoverflow.com/a/3841804
2. http://technet.microsoft.com/en-us/library/ms189499.aspx (Logical Processing Order of the SELECT statement)

The correct query should be:
```
SELECT c0_.Country AS sclr0
FROM Continent c0_ WITH (NOLOCK)
WHERE c0_.Country = 38
GROUP BY c0_.Country
```