Before this change the WebSocketTransportHandler passed
Collections.emptyMap as attributes to the HandshakeHandler because
it didn't matter what attributes the underlying WebSocketSession has
since it is wrapped by the SockJsSession and that's what exposed for
use everywhere.
This change has the WebSocketTransportHandler passing the attributes
from the SockJsSession instead since it's more accurate for the
underlying WebSocketSession to have access to the same map instance
and it allows the HandshakeHandler to change the attributes even if
it doesn't need to do that today.
Issue: SPR-12716