Before this patch:

  $ bin/mruby -e 'p 3.to_s(2)'
  trace (most recent call last):
    [0] -e:1
  -e:1: 'to_s': wrong number of arguments (1 for 0) (ArgumentError)

After this patch:

  $ bin/mruby -e 'p 3.to_s(2)'
  "11"