fix: correct PHP notice on register-rest-route in WordPress v5.5.0

(cherry picked from commit b4ec45b)

Author: rkaiser0324

src/Plugin.php

@@ -164,6 +164,7 @@ private function registerTestEndpoint() {
                 array(
                     'methods' => 'POST',
                     'callback' => '\Rollbar\Wordpress\Plugin::testPhpLogging',
+                    'permission_callback' => '__return_true',
                     'args' => array(
                         'server_side_access_token' => array(
                             'required' => true