HttpStatusCode to implement RFC 6585

[ghost]

Please implement complete list of HTTP status codes. See https://tools.ietf.org/html/rfc6585 for many status codes that are missing in:

https://github.com/dotnet/corefx/blob/master/src/System.Net.Primitives/src/System/Net/HttpStatusCode.cs

Ignoring the Unofficial codes section from this list: https://en.wikipedia.org/wiki/List_of_HTTP_status_codes, we basically should have all status codes covered in HttpStatusCode enum.

Ex, Azure DocumentDB server returns 429 (Too Many Requests) for which i have to check integral value of HttpResponseMessage.StatusCode in HttpClient.

[davidsh]

This issue is basically a duplicate of this: #15650

Thanks!