[ruby] Allow boolean for discriminator value in oneOf handling

Created by: ykitamura-mdsol

This must be a very rare case but we are using a boolean property as discriminator:

    CarOrCustomCar:
      oneOf:
        - $ref: '#/components/schemas/Car'
        - $ref: '#/components/schemas/CustomCar'
      discriminator:
        propertyName: custom_flag
        mapping:
          true: '#/components/schemas/CustomCar'
          false: '#/components/schemas/Car'

    CustomCar:
      properties:
         :
        custom_flag:
          type: boolean

    Car:
      properties:
         :
        custom_flag:
          type: boolean

The OpenAPI specification says:

Mapping keys MUST be string values, but tooling MAY convert response values to strings for comparison.

So I think we can modify to handle boolean values property for discriminator.

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