fix: handle single function middleware files

close #303

playground/nuxt.config.js

@@ -23,7 +23,8 @@ export default {
   },
   serverMiddleware: [
     { path: '/express', handler: '~/server/index.ts' },
-    '~/server/object.ts'
+    '~/server/object.ts',
+    '~/server/function.ts',
   ],
   publicRuntimeConfig: {
     moduleName: "@nuxtjs/storybook",

playground/server/function.ts

@@ -0,0 +1,10 @@
+// ~/server/function.ts
+
+export default (req, res, next) => {
+    if(req.url === '/express3') {
+        res.write("Works")
+        res.end()
+        return
+    }
+    next()
+}

storybook/middleware.js

@@ -1,15 +1,16 @@
 const { middlewares } = require('../<%= relativeToBuild(options.moduleDir + "/runtime/middlewares.js") %>')
 module.exports = function (app) {
   const addServerMiddleware = (middleware) => {
-    // Directly use functions
-    if (typeof middleware === 'function') {
-      return app.use(middleware);
-    }
-
     if (typeof middleware === 'string') {
       middleware = require(middleware)
       middleware = middleware.default || middleware
     }
+
+    // Directly use functions
+    if (typeof middleware === 'function') {
+      return app.use(middleware);
+    }
+
     const { handler, handle, path, route } = middleware
 
     let _route = path || route || '/'