mark a response as closed on the "close" event

references josephg/node-browserchannel#20

ignore ```res.write```, ```res.end``` calls if response is closed

I added ```unless responseClosed``` to enough lines for the tests to pass and my sharejs server to work with node version v0.8.20.  Not being too familiar with this codebase, I'm not sure if all the other res.write calls should have this clause as well.