Use "close" event on res rather than attaching an event handler to th…

…e socket

lib/controllers/proxy.js

@@ -198,7 +198,7 @@ module.exports = function(options) {
         // encoding : null means "body" passed to the callback will be raw bytes
 
         var proxiedRequest;
-        req.socket.once('close', function() {
+        res.once('close', function() {
             if (proxiedRequest && !res.writableEnded) {
                 proxiedRequest.abort();
             }