Replace _headerSent with headersSent

According to the documentation http://nodejs.org/api/http.html#http_response_headerssent, we can use `headersSent`.

I may be wrong on this one, but this just bit us in production where we had a custom `server.on('uncaughtException')` listener, and the `res._headerSent` were not working. Updating this to `headersSent` fixed the problem.

It looks to be available under node 0.10 only, if we want to maintain compatibility maybe we should use `|| res._headerSent || res.headersSent`.

I can't find any reference in the code where you set the `_headerSent` value, so I figured you were using a deprecated value from a low-level library.