fix: use spread operator instead of Ember.assign

Ember.assign polyfill is deprecated https://deprecations.emberjs.com/v4.x/#toc_ember-polyfills-deprecate-assign

addon/helpers/page-title.js

@@ -1,7 +1,6 @@
 import { inject as service } from '@ember/service';
 import Helper from '@ember/component/helper';
 import { guidFor } from '@ember/object/internals';
-import { assign } from '@ember/polyfills';
 
 /**
   `{{page-title}}` helper used to set the title of the current route context.
@@ -23,10 +22,11 @@ export default class PageTitle extends Helper {
   }
 
   compute(params, _hash) {
-    let hash = assign({}, _hash, {
+    let hash = {
+      ..._hash,
       id: this.tokenId,
       title: params.join(''),
-    });
+    };
 
     this.tokens.push(hash);
     this.tokens.scheduleTitleUpdate();

addon/services/page-title-list.js

@@ -2,7 +2,6 @@ import { getOwner } from '@ember/application';
 import { scheduleOnce } from '@ember/runloop';
 import Service, { inject as service } from '@ember/service';
 import { isEmpty } from '@ember/utils';
-import { assign } from '@ember/polyfills';
 import { assert } from '@ember/debug';
 
 let isFastBoot = typeof FastBoot !== 'undefined';
@@ -164,7 +163,7 @@ export default class PageTitleListService extends Service {
         }
         let lastToken = group[0];
         if (lastToken) {
-          token = assign({}, token);
+          token = { ...token };
           token.separator = lastToken.separator;
         }
         group.unshift(token);