bugfix: Correct reference to websocket room name
bugfix: Correct reference to websocket room name

When inside the onmessage function, `this` no longer refers to the
websocket node. It needs to be accessed with `that`, which is where we
stored it previously.

jagenjo/litegraph.js,jagenjo/litegraph.js,jagenjo/litegraph.js,jagenjo/litegraph.js,jagenjo/litegraph.js