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.
fix#171
Several names and HTTP identifiers are renamed to be
more consistent, self-explanatory, and concise:
* "Fields" is a collection of HTTP header fields (rfc7230 section 3.2)
* "Header" is the Start Line plus Fields. Another way to look at it is,
the HTTP message minus the body.
* `basic_fields` replaces `basic_headers`
* `fields` replaces `headers`
* `Fields` replaces `Headers` in template parameter lists
* `header` replaces `message_headers`
* `header::fields` replaces `message_headers::fields`
The changes are cosmetic and do not affect run-time behavior.
