[DOC] lost response.timestamp when running an example

[cainiaoDJ]

Describe the bug
read _examples\mvc\error-handler-preflight\main.go code, there is response.Timestamp in response object, but it not appear in the response when I call "http://lcoalhost:8080/1"
I think the field of timestamp should appear in returns when response.ID > 0, otherwise this is a meaningless field。

To Reproduce

cd _examples/mvc/error-handler-preflight
go run main.go
curl --location --request GET 'http://127.0.0.1:8080/1'

Expected behavior

{
    "id": 1,
    "data": {
        "id": 1
    },
    "timestamp": 1609227854
}

Desktop (please complete the following information):

• OS: windows10

iris.Version

• master