Changed Query.get_columns() to quote the 'AS' column names in an extra_select...

Changed Query.get_columns() to quote the 'AS' column names in an extra_select situation, to match pre-queryset-refactor behavior. Added unit tests that verify this and provide an example

git-svn-id: http://code.djangoproject.com/svn/django/trunk@7502 bcc190cf-cafb-0310-a4f2-bffc1f526a37