Why HttpRequest.HttpMethod is string instead of Enum?

In the Reference of HttpRequest.HttpMethod of .NET Framework, request type is declared with System.String type.

In RFC 2616 all HTTP request methods are declared (e.g. POST, GET, PUT, DELETE...).

There's also similar behavior in HttpWebRequest and WebRequest classes of .NET.

Java has the similar approach on HttpURLConnection#setRequestMethod(String) method.

Why do these language designers do not consider implementing an enum for those HTTP methods?

Do you have an idea?