Use Laravel's class-based Str and Arr helpers

Starting with version 5.9, the global funtions will be deprecated.

* https://laravel-news.com/laravel-5-8-deprecates-string-and-array-helpers
* https://github.com/laravel/framework/pull/26898

src/Api/Controller/CreateUserController.php

@@ -14,6 +14,7 @@ namespace Flarum\Api\Controller;
 use Flarum\Api\Serializer\CurrentUserSerializer;
 use Flarum\User\Command\RegisterUser;
 use Illuminate\Contracts\Bus\Dispatcher;
+use Illuminate\Support\Arr;
 use Psr\Http\Message\ServerRequestInterface;
 use Tobscure\JsonApi\Document;
 
@@ -43,7 +44,7 @@ class CreateUserController extends AbstractCreateController
     protected function data(ServerRequestInterface $request, Document $document)
     {
         return $this->bus->dispatch(
-            new RegisterUser($request->getAttribute('actor'), array_get($request->getParsedBody(), 'data', []))
+            new RegisterUser($request->getAttribute('actor'), Arr::get($request->getParsedBody(), 'data', []))
         );
     }
 }