Emitted when the server has started reading a new request.
message will be completely blank; not even the
Request-Line will have been read yet. About the only thing
you can usefully do with it is connect to its signals.
If the request is read successfully, this will eventually
be followed by a [signal@Server::request_read signal]. If a
response is then sent, the request processing will end with
SoupServer::request-finished signal. If a network error
occurs, the processing will instead end with
The default handler is called before the handlers added via
The new message.
The data is owned by the caller of the function.