Error codes must be strings

[halilim]

According to the spec, codes in the exceptions must be strings. Currently generated errors aren't valid against the JSON API Schema. See cerebris/jsonapi-resources#675. An example:

{
  "errors": [
    {
      "title": "Bad Request",
      "detail": "This request is not supported.",
      "code": 400,
      "status": "400"
    }
  ]
}

[tiagopog]

Thanks for the feedback, @halilim. I may come up with a PR in few hours.