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#124
The http::header data members "method", "url", and "reason"
are changed from data members, to pairs of get and set
functions which forward the call to the Fields type used
to instantiate the template.
Previously, these data members were implemented using
std::string. With this change, the implementation of the
Fields type used to instantiate the template is now in
control of the representation of those values. This permits
custom memory allocation strategies including uniform use of
the Allocator type already provided to beast::http::basic_fields.
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.
Vinnie Falco