Merge pull request #74 from koajs/req-res-next

pass an optional `next` to app.callback()

lib/application.js

@@ -88,10 +88,10 @@ app.callback = function(){
   var gen = compose(mw);
   var self = this;
 
-  return function(req, res){
+  return function(req, res, next){
     var ctx = new Context(self, req, res);
 
-    co.call(ctx, gen)(ctx.onerror);
+    co.call(ctx, gen)(next || ctx.onerror);
   }
 };