Replace kind_of with instance_of for String#to_s

Similar fix to b69b21b320c70887b8991c6b21a551d605335c95 for
core/array/to_a_spec.rb

If you have an instance of a subclass of String, calling `:to_s` on it
should return an instance of String rather than of the subclass.
Merge pull request #257 from scooter-dangle/patch-1

Replace kind_of with instance_of for String#to_s