Switch to afterResolving when registering service provider (fedeisas#90)

Author: TheoGibbons

src/LaravelMailCssInlinerServiceProvider.php

@@ -30,7 +30,7 @@ public function register()
             return new CssInlinerPlugin($app['config']->get('css-inliner'));
         });
 
-        $this->app->extend('mail.manager', function (MailManager $mailManager) {
+        $this->app->afterResolving('mail.manager', function (MailManager $mailManager) {
             $mailManager->getSwiftMailer()->registerPlugin($this->app->make(CssInlinerPlugin::class));
             return $mailManager;
         });