Fixed missing select statement in queries (#1223)

If you don't specify select columns in where subqueries/union statements select part of query is missing. See bug and my comment https://github.com/laravel/framework/issues/1223#issuecomment-17559727 for explanation.

Solution: If no columns have been specified for the select statement, we will set them to standard default of retrieving all of the columns on the table using the "wildcard" column character.