Fix cookie test with newer versions of Node

An HTTP header must not be set to an `undefined` value.

script/server

@@ -104,7 +104,10 @@ var routes = {
     if (params.name) {
       cookie = querystring.parse(req.headers['cookie'], '; ')[params.name];
     }
-    res.writeHead(200, {'Content-Type': 'text/plain', 'Set-Cookie': setCookie});
+    res.writeHead(200, {
+      'Content-Type': 'text/plain',
+      'Set-Cookie': setCookie || ''
+    });
     res.end(cookie);
   },
   '/headers': function(res) {