Logstash Mutate Convert Not Working

Hi All,

I've created logstash ruby filter like this (ignore my val.each loop, I'm tring to convert all field to string):
image

However, I still cannot convert my field to string (the field is showed as date field):

My data is a dict and array as the value:

{
"logs": [
    {
      "@odata.id": "/redfish/v1/Systems/1/LogServices/IML/Entries/55",
      "Created": "2018-12-09T09:08:04Z",
      "Id": "55",
      "Message": "Automatic Operating System Shutdown Initiated Due to Overheat Condition",
      "Name": "Integrated Management Log",
      "Severity": "Critical"
    },
    {
      "@odata.id": "/redfish/v1/Systems/1/LogServices/IML/Entries/143",
      "Created": "2019-02-03T08:04:36Z",
      "Id": "143",
      "Message": "Automatic Operating System Shutdown Initiated Due to Overheat Condition",
      "Name": "Integrated Management Log",
      "Severity": "Critical"
    }]
}

How do I convert the logs.Created to string?
I've tried using mutate convert filter too but it doesn't seems to be working