Update to drupal 8.0.0-rc1. For more information, see https://www.drupal.org/node/2582663

vendor/guzzlehttp/psr7/tests/bootstrap.php

@@ -0,0 +1,11 @@
+<?php
+namespace GuzzleHttp\Tests\Psr7;
+
+require __DIR__ . '/../vendor/autoload.php';
+
+class HasToString
+{
+    public function __toString() {
+        return 'foo';
+    }
+}