Fix isString() method in yaml_cpp_adapter

[domire8]

Hi @tristanpenman, @psigen,

First of all, thanks for the amazing work in this library, super cool!

I've tried the yaml_cpp_adapter to validate a YAML file with a custom JSON schema and experience runtime errors when the YAML app included arrays of objects, for example

start:
  test:
    - { name: one, value: two }
    - { name: ten, value: twenty }

and I seem to have found the bug that was causing these runtime errors. While it's true that each element in a YAML should be considered as string (hence probably the return true in the isString() method), this should only return true when the element is actually a scalar value and not a sequence or a map.

Happy to hear your feedback.

[tristanpenman]

Hey @domire8, thanks for the fix. This seems like a reasonable change to me 👍

[psigen]

Thanks for finding and fixing the issue so quickly!