[JS] Refactor oneOf validation logic, add more tests

[wing328]

• Refactor oneOf validation logic (both constructor, constructFromObject share the same validation code)
• Add more tests

PR checklist

• Read the contribution guidelines.
• Pull Request title clearly describes the work in the pull request and Pull Request description provides details about how to validate the work. Missing information here may result in delayed response from the community.
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  ./bin/utils/export_docs_generators.sh
  

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cc @CodeNinjai (2017/07) @frol (2017/07) @cliffano (2017/07)