Update to Drupal 8.0.0-rc3. For more information, see https://www.drupal.org/node/2608078

vendor/jcalderonzumba/mink-phantomjs-driver/src/HeadersTrait.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace Zumba\Mink\Driver;
+
+/**
+ * Class HeadersTrait
+ * @package Zumba\Mink\Driver
+ */
+trait HeadersTrait {
+
+  /**
+   * Gets the current request response headers
+   * Should be called only after a request, other calls are undefined behaviour
+   * @return array
+   */
+  public function getResponseHeaders() {
+    return $this->browser->responseHeaders();
+  }
+
+  /**
+   * Current request status code response
+   * @return int
+   */
+  public function getStatusCode() {
+    return $this->browser->getStatusCode();
+  }
+
+  /**
+   * The name say its all
+   * @param string $name
+   * @param string $value
+   */
+  public function setRequestHeader($name, $value) {
+    $header = array();
+    $header[$name] = $value;
+    //TODO: as a limitation of the driver it self, we will send permanent for the moment
+    $this->browser->addHeader($header, true);
+  }
+
+}