The verb interfaces now use verb::unknown instead of
boost::optional<verb> == boost::none to indicate that
the request-method is an unrecognized string.
The http::header interface is modified to focus more on the
verb enum rather than the string. For recognized verbs, the
implementation stores an integer instead of the string.
Unknown verbs are still stored as strings.
* header::method now returns a verb
* header::method_string returns the method text
fix#397
method enum class is added to represent all known request methods.
Functions are provided to convert from strings to and from the method
enumeration.
The fields container is modified to also work with the enum.
