fix: use correct mime type key

Author: digitalsadhu

lib/middleware.js

@@ -82,7 +82,7 @@ module.exports = class Middleware extends EventEmitter {
 
     js() {
         return (req, res, next) => {
-            res.type('javascript');
+            res.type('js');
             this.writers.js
                 .bundle()
                 .on('error', next)