Add HttpStatusClass

Related: #3157

Motivation:

It should be convenient to have an easy way to classify an
HttpResponseStatus based on the first digit of the HTTP status code, as
defined in the RFC 2616:

- Information 1xx
- Success 2xx
- Redirection 3xx
- Client Error 4xx
- Server Error 5xx

Modification:

- Add HttpStatusClass
- Add HttpResponseStatus.codeClass() that returns the class of the HTTP
  status code
- Remove HttpResponseStatus.isInformational()

Result:

It's easier to determine the class of an HTTP status