http: use socket.once, not socket.on

Register the 'close' event listener with .once(), not .on().

It doesn't matter in the grand scheme of things because the listener
doesn't keep references to any heavy-weight objects but using .once()
for a oneshot listener is something of a best practice.

lib/http.js

@@ -515,7 +515,7 @@ OutgoingMessage.prototype._writeRaw = function(data, encoding) {
       var timer = setTimeout(function() {
         socket.emit('close');
       });
-      socket.on('close', function() {
+      socket.once('close', function() {
         clearTimeout(timer);
       });
     }