PHP model field validation broken on oneOf

Created by: dacgray

Description

 bar:
      properties:
        field:
          type: string
          enum: [a,b]
 ray:
      properties:
        field:
          type: string
          enum: [c,d]

MyObject:
      properties:
        foo:
          type: array
          items:
            oneOf:
              - $ref: '#/components/schemas/bar'
              - $ref: '#/components/schemas/ray'
              ```

ObjectSerializer::deserialize (src/Http/ObjectSerializer.php:478) fails:

getFieldAllowableValues & getOperatorAllowableValues do not combine all options and validation fails.

It instead only uses enum values from the last item (ray) for validation

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