test: fix test-http-extra-response flakiness

It can happen that the extra response is to be sent in a different chunk
from the rest of the data. At this moment, the client might have already
closed the socket causing an `ECONNRESET` error.

PR-URL: https://github.com/nodejs/node/pull/4979
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

test/parallel/test-http-extra-response.js

@@ -37,6 +37,10 @@ var server = net.createServer(function(socket) {
       socket.end(fullResponse);
     }
   });
+
+  socket.on('error', function(err) {
+    assert.equal(err.code, 'ECONNRESET');
+  });
 });