bugfix : Cannot res.send() in on End event

bug fix : #73

Author: youtakanke

lib/filehandler.js

@@ -10,7 +10,7 @@ module.exports = function (middleware, options) {
             if (redirect) {
                 files = {files: result};
                 res.redirect(redirect.replace(/%s/, encodeURIComponent(JSON.stringify(files))));
-            } else {
+            } else if (!res.finished){
                 res.set({
                     'Content-Type': (req.headers.accept || '').indexOf('application/json') !== -1
                         ? 'application/json'