web中间件从5.2.27版本以后默认全局加载，不需要自己手动载入，如果自己手动重复载入，会导致session无法加载等情况。
下面是Laravel默认为我们提供的web和api中间件组的代码：
protected $middlewareGroups = [
'web' => [
\App\Http\Middleware\EncryptCookies::class,
\Illuminate\Cookie\Middleware\AddQueuedCookiesToResponse::class,
\Illuminate\Session\Middleware\StartSession::class,
\Illuminate\View\Middleware\ShareErrorsFromSession::class,
\App\Http\Middleware\VerifyCsrfToken::class,
],
'api' => [
'throttle:60,1',
],
];
