timestamp issue in codegen

openapi.oas2.yml

swagger: '2.0'
info:
  title: Example to describe swagger codegen issue
  description: |
    Example Definition with Timestamp class as `java.sql.Timestamp` instead of custom model.
  version: v1

paths:
  /foo:
    post:
      operationId: MakeFoo
      description: ok
      parameters:
        - name: body
          in: body
          schema:
            type: object
            properties:
              timestamp:
                $ref: '#/definitions/Timestamp'
      responses:
        default:
          description: Successful operation
definitions:
  Timestamp:
    type: object
    properties:
      epochMillis:
        description: Milliseconds since the Unix epoch
        type: string
        format: int64