AllowHttpStatus overload should take a set of ints, not HttpStatusCodes

[tmenier]

Flurl (ever opinionated) generally favors expressing HTTP status codes as integers, rather than using .NET's HttpStatusCode enum. While updating the docs I was reminded that it breaks from that convention here:

clientOrRequest.AllowHttpStatus(HttpStatusCode.Conflict, HttpStatusCode.NotFound)...;

Although a breaking change, I'd like to switch it to use ints for 4.0 for consistency with other things:

clientOrRequest.AllowHttpStatus(409, 404)...;