* header::result is a family of functions to replace header::status
* header interface now uses status enum and also ints
* reason-phrase is no longer stored unless the user explicitly
requests it.
* When serializing, the standard reason is used for the
corresponding status code unless the user has changed it.
fix#398
A new enum status is added for the status code.
The function obsolete_reason returns default reason phrasing.
If a response has an empty reason, the serializer will
automatically insert the default reason phrase for the
status code.