ref: Use a named function for the middleware over an anonymous (#424)

It's making the investigation of performance issues easier.

Author: oliviertassinari

lib/client.js

@@ -532,7 +532,7 @@ extend(Raven.prototype, {
 
   requestHandler: function() {
     var self = this;
-    return function(req, res, next) {
+    return function ravenMiddleware(req, res, next) {
       self.context({req: req}, function() {
         domain.active.add(req);
         domain.active.add(res);