Actions Required:
* Change calls to msg.prepare to msg.prepare_payload. For messages
with a user-defined Fields, provide the function prepare_payload_impl
in the fields type according to the Fields requirements.
* Container interface more closely matches std::vector
* While preserving the invariant that duplicate fields
with the same case-insensitive name have their order
preserved.
* 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.
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.
