It was found that a feature to check and add ruby2_keywords flag to an
existing Hash is needed when arguments are serialized and deserialized.
It is possible to do the same without explicit APIs, but it would be
good to provide them as a core feature.
https://github.com/rails/rails/pull/38105#discussion_r361863767
Hash.ruby2_keywords_hash?(hash) checks if hash is flagged or not.
Hash.ruby2_keywords_hash(hash) returns a duplicated hash that has a
ruby2_keywords flag,
[Bug #16486]