Fix test failure on PHP 7.2 caused by Guzzle.

We do not directly depend on Guzzle, but our tests are failing on PHP
7.2 due to a property not being correctly initialized.

Ref. guzzle/guzzle#1686

Author: pfrenssen

composer.json

@@ -13,6 +13,7 @@
         "consolidation/robo": "~1.3",
         "drush/drush": "~9.4",
         "drupal-composer/drupal-scaffold": "~2.2",
+        "guzzlehttp/guzzle": "~6.3",
         "instaclick/php-webdriver": "^1.4.5",
         "nikic/php-parser": "~3.0|~4.0",
         "openeuropa/code-review": "^1.0.0-alpha4",