NH-3800 - Modifed the GroupBySelectClauseRewriter to allow member access of convert expressions

* This allows the group element selector be cast to the expected type, simplifying the select clause.

Author: PleasantD

src/NHibernate/Linq/GroupBy/GroupBySelectClauseRewriter.cs

@@ -59,7 +59,8 @@ protected override Expression VisitMemberExpression(MemberExpression expression)
 				return base.VisitMemberExpression(expression);
 			}
 
-			if (elementSelector is NewExpression && elementSelector.Type == expression.Expression.Type)
+			if ((elementSelector is NewExpression || elementSelector.NodeType == ExpressionType.Convert) 
+                && elementSelector.Type == expression.Expression.Type)
 			{
 				//TODO: probably we should check this with a visitor
 				return Expression.MakeMemberAccess(elementSelector, expression.Member);