Meet interface expectation for active record.

Count must still accept a second parameter.
https://github.com/rails/rails/blob/0ef8a244231c355cab7f4d032efed51ef5ddfe99/activerecord/lib/active_record/relation/calculations.rb#L30

I'm not entirely sure why this method is here. I assume its for a
reason.
Merge pull request #600 from freerunningtech/master

Meet interface expectation for active record.