Filter out non-strings (serverMiddleware watch fix) (#3771)

* Filter out non-strings

* Add serverMiddleware function

* Add serverMiddleware function (2)

* Add serverMiddleware function (3)

lib/builder/builder.js

@@ -643,7 +643,9 @@ export default class Builder {
 
   watchServer() {
     const nuxtRestartWatch = _.concat(
-      this.options.serverMiddleware.map(this.nuxt.resolveAlias),
+      this.options.serverMiddleware
+        .filter(i => typeof i === 'string')
+        .map(this.nuxt.resolveAlias),
       this.options.watch.map(this.nuxt.resolveAlias),
       path.join(this.options.rootDir, 'nuxt.config.js')
     )

test/fixtures/cli/nuxt.config.js

@@ -1,5 +1,8 @@
 export default {
-  serverMiddleware: ['~/middleware.js'],
+  serverMiddleware: [
+    '~/middleware.js',
+    (req, res, next) => next()
+  ],
   watch: ['~/custom.file'],
   hooks(hook) {
     hook('listen', (server, { port, host }) => {