serDes does not work in additionalProperties objects

[leonardo-speranzon]

Describe the bug
The serialization of date-time not work if the field is in a additionalProperties object

To Reproduce

The openapi definition

  /test:
    get: 
      responses:
        '200':
          description: foo
          content:
            application/json:
              schema:
                type: object
                properties:
                  good_date:
                    type: string
                    format: date-time
                  other_info:
                    type: object
                    additionalProperties:
                      type: object
                      properties:
                        bad_date:
                          type: string
                          format: date-time

Express server implementation

app.use(
    OpenApiValidator.middleware({
        apiSpec: './openapi.yaml',
        validateRequests: true, // (default)
        validateResponses: true, // false by default
        serDes: [
            OpenApiValidator.serdes.date,
            OpenApiValidator.serdes.dateTime,
        ]  // the default ones
    }),
)

app.get('/test', (req:Request,res:Response,next:NextFunction)=>{
    return res.json({
        good_date: new Date(),
        other_info: {
            something:{
                bad_date: new Date()
            }
        }
    })
})

Actual behavior
The serializer does not convert the bad_date from date to string and so the validator produce an error

  "message": "/response/other_info/something/bad_date must be string",
    "errors": [
        {
            "path": "/response/other_info/something/bad_date",
            "message": "must be string",
            "errorCode": "type.openapi.validation"
        }
    ]

Expected behavior
The response should be validated succesfully