http_server: prefinish vs finish

Do not detach the socket from the response until all data is actually
sent to the other side.

See: #1373

lib/_http_server.js

@@ -429,7 +429,7 @@ function connectionListener(socket) {
 
     // When we're finished writing the response, check if this is the last
     // respose, if so destroy the socket.
-    res.on('prefinish', resOnFinish);
+    res.on('finish', resOnFinish);
     function resOnFinish() {
       // Usually the first incoming element should be our request.  it may
       // be that in the case abortIncoming() was called that the incoming