tornado, fix post json error using request body

[zyfxn]

1. fix post json error using request body: json data is bytes, not string, cause validation failed.
2. support ref in request body:

    post:
      summary: Creates a new user.
      consumes:
        - application/json
      parameters:
        - in: body
          name: user
          description: The user to create.
          schema:
            $ref: '#/definitions/User'     # <----------