Added support for calling count_by without a block

Good thing Rubinius is in Ruby, I learned about `to_enum` from [what they did with `group_by`](https://github.com/rubinius/rubinius/blob/master/kernel/common/enumerable18.rb#L45).

I ran the spec with 1.9.2-p290, 1.9.3-p429 and 2.0.0-p195.