make session id accessable by handler

Though SockJS and all sessions could be accessed by handler with the custom session_factory constructor, current/self session id still has no way to be accessed by handler.
SockJS->new(
		session_factory	=> sub {
			my $parent = shift;
			weaken $parent;
			SockJS::Session->new( parent => $parent)
		},
);

By having this change, handlers can access current/self session id if they use the custom session_factory constructor below.
SockJS->new(
		session_factory	=> sub {
			my $parent = shift;
			my $id = shift;
			weaken $parent;
			SockJS::Session->new( parent => $parent, id => $id)
		},
);

Knowing current/self session id is a must-have in many situations, so passing it (or PATH_INFO/REQUEST_URI, or $env even better) to handler in some way might be a good idea, to avoid too much dirty hacks in the codebase.