Convert property names to camel case

[MAILPOET-1796]

tests/integration/API/JSON/APITest.php

@@ -25,7 +25,7 @@ require_once('APITestNamespacedEndpointStubV2.php');
 
 class APITest extends \MailPoetTest {
   public $api;
-  public $wp_user_id;
+  public $wpUserId;
   /** @var Container */
   private $container;