@jc00ke @leocassarani @dbussink
Thank you for thinking about visibility/arity specs!
I'll remove these specs from this commit.

And I'll add visibility specs if the visibility is not public or different by versions.
(But, as far as I know, builtin protected methods are nothing.)

@dbussink
Should I add visibility specs if the specs are sharing?
Visibilities are broken by `.send(@method)`.
I know two easy ways.

  * write public spec
  * use public_send

And I think public specs are better than public_send for keeping compatibility.