Arguments of `#sum` does not match with other shortcuts methods
(count, average, minimum, and maximum).
This commit fix these two points:

* call `super` with only block arguments
  First argument of `super` method, `Enumerable#sum`, is `identity`
  and first argument of `AR::Calculations#sum` is `column_name`.
  `Enumerable#sum` does not expect `column_name` to be passed.
* Change first argument of `sum` from array arguemnt to single
  argument to match other shortcuts methods. When `sum` accept
  array arguemnt, user can pass multi arguments and an exception is
  raised from `calculate`.