Fix oneOf schema properties

Created by: lwlee2608

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fix #4515 (closed)

Given the following schema, Area should have two properties fields tacs and areaCode. The generated model Area shouldn't really care if they are defined as oneOf. It is up to the validation logic to validate that.

    Area:
      type: object
      oneOf:
        - required:
          - tacs
        - required:
          - areaCode
      properties:
        tacs:
          type: array
          items:
            $ref: '#/components/schemas/Tac'
          minItems: 1
        areaCode:
            $ref: '#/components/schemas/AreaCode'

cc @wing328 @jimschubert @cbornet @ackintosh @jmini @etherealjoy