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
```

lib/Doctrine/ORM/Query/SqlWalker.php

@@ -21,6 +21,7 @@ namespace Doctrine\ORM\Query;
 
 use Doctrine\DBAL\LockMode;
 use Doctrine\DBAL\Types\Type;
+use Doctrine\DBAL\Platforms\SQLServerPlatform;
 use Doctrine\ORM\Mapping\ClassMetadata;
 use Doctrine\ORM\Query;
 use Doctrine\ORM\Query\QueryException;
@@ -1624,7 +1625,11 @@ class SqlWalker implements TreeWalker
 
         // ResultVariable
         if (isset($this->queryComponents[$groupByItem]['resultVariable'])) {
-            return $this->walkResultVariable($groupByItem);
+            if ($this->platform instanceof SQLServerPlatform) {
+                return $this->walkPathExpression($this->queryComponents[$groupByItem]['resultVariable']->pathExpression);
+            } else {
+                return $this->walkResultVariable($groupByItem);
+            }
         }
 
         // IdentificationVariable