Providing node should not be required on custom event fire()

It doesn't seem like submitting back the node should be required on custom events. I know I was perplexed with cryptic 'undefined does not have property _ractive' message ( generated [here](https://github.com/RactiveJS/Ractive/blob/0.4.0/src/render/DomFragment/Element/initialise/addEventProxies/addEventProxy.js#L168) ) .

I created a closure that supplies the node, if it is missing. (Is there a case for not using the original node?)