common options for both:
=over
=item * C<subprotocols> - A list of HTTP tokens (e.g., C<wamp.2.json>)
that stand for subprotocols that this endpoint can use via the WebSocket
connection.
=item * C
p_token = I<OBJ>->get_subprotocol()
Returns the negotiated subprotocol’s token (e.g., C<wamp.2.json>).
=cut
sub get_subprotocol {
my $self = shift;
if (!$self->{'_no_use_legacy'}) {