set `headerSent` on `header` event

Using the [compress middleware](http://www.senchalabs.org/connect/compress.html) from connect, I found that connect middlewares check if the header has been sent by checking for the `headerSent` property on the response stream. The other middleware that uses this is [timeout](http://www.senchalabs.org/connect/timeout.html).

Since neither `headerSent` or `_emittedHeader` are in the [public API](http://nodejs.org/api/http.html#http_class_http_serverresponse), I think it's better to make union and connect work with each other.