fix AttributeCustomFields return null in REST/JSON service

[annProg]

For example, service_details is AttributeCustomFields，in rest/json service, it will return null

{
  "objects": {
    "UserRequest::3979": {
      "code": 0,
      "message": "",
      "class": "UserRequest",
      "key": "3979",
      "fields": {
        "service_details": null
      }
    }
  },
  "code": 0,
  "message": "Found: 1"
}

After fix:

{
  "objects": {
    "UserRequest::3979": {
      "code": 0,
      "message": "",
      "class": "UserRequest",
      "key": "3979",
      "fields": {
        "service_details": {
          "legacy": "",
          "extradata_id": "3898",
          "_template_name_": "域名申请模板",
          "template_id": 7,
          "template_data": "",
          "user_data": {
            "name": "a.b.com",
            "applicationsolution_list": "22108",
            "tips_record_id": "public",
          },
          "user_data_objclass": {
            "applicationsolution_list": "ApplicationSolution"
          },
          "current_template_id": 7,
        }
      }
    }
  },
  "code": 0,
  "message": "Found: 1"
}