Fix broken count behavior

My patch removing deprecated count(condtions, joins) - http://dev.rubyonrails.org/ticket/8993 got committed today and it broke Model.count(:column) behavior, which was undocumented feature. So I've submitted a new patch fixing it and adding some tests - http://dev.rubyonrails.org/ticket/8999

Please comment/review.

Thanks, Pratik