refactor: use object spread operator

lib/module.js

@@ -24,7 +24,12 @@ module.exports = function module (moduleOptions) {
     defaults: {}
   }
 
-  const options = Object.assign({}, defaults, this.options.sitemap, moduleOptions)
+  const options = {
+    ...defaults,
+    ...this.options.sitemap,
+    ...moduleOptions
+  }
+
   options.pathGzip = options.gzip ? `${options.path}.gz` : options.path
 
   // sitemap-routes.json is written to dist dir on "build" mode
@@ -155,11 +160,11 @@ function createSitemap (options, routes, req) {
   sitemapConfig.hostname = options.hostname ||
     (req && `${isHTTPS(req) ? 'https' : 'http'}://${req.headers.host}`) || `http://${hostname()}`
 
-  routes = routes.map(route => Object.assign({}, options.defaults, route))
+  routes = routes.map(route => ({ ...options.defaults, ...route }))
 
   // Enable filter function for each declared route
   if (typeof options.filter === 'function') {
-    routes = options.filter({ routes, options: Object.assign({}, options, sitemapConfig) })
+    routes = options.filter({ routes, options: { ...options, ...sitemapConfig } })
   }
 
   // Set urls and ensure they are unique