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