clone options to prevent unwanted side-effects

fixes with-config test that runs nuxt twice with same nuxt.config.js file

lib/nuxt.js

@@ -11,9 +11,12 @@ import Server from './server'
 import Defaults from './defaults'
 
 export default class Nuxt extends Tapable {
-  constructor (options = {}) {
+  constructor (_options = {}) {
     super()
 
+    // Clone options to prevent unwanted side-effects
+    const options = Object.assign({}, _options)
+
     // Normalize options
     if (options.loading === true) {
       delete options.loading

test/with-config.test.js

@@ -111,4 +111,5 @@ test.after('Should be able to start Nuxt with build done', async t => {
   config.rootDir = rootDir
   config.dev = false
   nuxt = new Nuxt(config)
+  await nuxt.init()
 })