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#195
This function returns `true` when the passed HTTP Request
indicates a WebSocket Upgrade. It does not validate the
contents of the fields: it just trivially accepts requests
which can only be a WebSocket Upgrade message.
Callers who wish to manually read HTTP requests in their
server implementation can use this function to determine if
the request should be routed to an instance of websocket::stream.
