Added HTTP Status codes from the RFCs 4918 and 7538

[0xdeafcafe]

These two RFCs are finalized, and rather useful that I feel we should have in the core framework.

7538 RFC Spec
4918 RFC Spec

[dnfclas]

Hi @0xdeafcafe, I'm your friendly neighborhood .NET Foundation Pull Request Bot (You can call me DNFBOT). Thanks for your contribution!

This seems like a small (but important) contribution, so no Contribution License Agreement is required at this point. Real humans will now evaluate your PR.

TTYL, DNFBOT;

[khellang]

Nice. Submitted 308 for Kestrel as well; aspnet/KestrelHttpServer#925

[Tratcher]

308 should be fine.

Are you actually using WebDAV? Or are you just adding them for completeness?

[0xdeafcafe]

I'm using the unprocessable_entity status code, as it's a good one to use if model validation fails, for instance.

I just added the entire RFC while I was at it for completeness sake. Currently I'm doing this, which I'd rather not (I know I could just extend StatusCodes myself, but I thought I'd just do a pull request instead):

if (!ModelState.IsValid)
    return Json(422 /* unprocessable_entity */, ModelState.ToDictionary());

var question = _lawsContext.Questions.FirstOrDefault(q => q.Id == questionId);
if (question == null)
    return Json(StatusCodes.Status404NotFound, new Error<object>("question_not_found"));

// Verify Module Id is real
var scope = _lawsContext.Scopes.FirstOrDefault(r => r.Id == (Guid) model.ScopeId);
if (scope == null)
    return Json(422 /* unprocessable_entity */, new Error<object>("scope_not_found"));

[Tratcher]

[Tratcher]

Rebased and merged. Thanks.

[xperiandri]

It would be also nice to have an UnprocessableEntityObjectResult class and UnprocessableEntity() method on Controller