Use Graby's http headers configuration for the authentication request

tests/SiteConfig/SiteConfigTest.php

@@ -37,6 +37,9 @@ class SiteConfigTest extends TestCase
             ],
             'username' => 'johndoe',
             'password' => 'unkn0wn',
+            'httpHeaders' => [
+                'user-agent' => 'Wallabag (Guzzle/5)',
+            ],
         ]);
 
         $this->assertInstanceOf(SiteConfig::class, $config);