Pass all message details to subscription event handler

[jbtbnl]

Using publisher disclosure with Thruway as server is challenging because not all message details are passed to the subscription event handler.

The Event object discloses a limited set of details to the event handler, but hides all (other) non-standard details. Therefore, the details authid and authrole as disclosed by Thruway are inaccessible in the event handler.

[oberstet]

See https://github.com/wamp-proto/wamp-proto/issues/57

[oberstet]

hides all (other) non-standard details

Custom (non-standard) attributes must start with x_ .. so the bug is not exposing all these, but not eg authroles which Thruway uses. That should probably be x_tw_authroles or something.

For example, Crossbar.io uses x_cb_node_id in the WELCOME message.