Merge branch '3.4' into 4.0

* 3.4: (22 commits)
  Add application/ld+json format associated to json
  [HttpFoundation] Fix false-positive ConflictingHeadersException
  remove flex-specific suggestion on 3.4
  Add check for SecurityBundle in createAccessDeniedException
  [WebServerBundle] Fix escaping of php binary with arguments
  Error handlers' $context should be optional as it's deprecated
  [Serializer] Correct typing mistake in DocBlock
  [HttpKernel] fix cleaning legacy containers
  Display n/a for sub-requests time when Stopwatch component is not installed
  Updating message to inform the user how to install the component
  [Config] Fix closure CS
  PHP CS Fixer: use PHPUnit Migration ruleset
  [Process] Skip false-positive test on Windows/appveyor
  Update MemcachedTrait.php
  [Bridge/PhpUnit] thank phpunit/phpunit
  allow auto_wire for SessionAuthenticationStrategy class
  [Process] Fix setting empty env vars
  Fixed 'RouterInteface' typo
  [Process] Dont use getenv(), it returns arrays and can introduce subtle breaks accros PHP versions
  [WebServerBundle] fix a bug where require would not require the good file because of env
  ...

Author: nicolas-grekas

Crawler.php

@@ -691,8 +691,8 @@ public function filterXPath($xpath)
      */
     public function filter($selector)
     {
-        if (!class_exists('Symfony\\Component\\CssSelector\\CssSelectorConverter')) {
-            throw new \RuntimeException('Unable to filter with a CSS selector as the Symfony CssSelector 2.8+ is not installed (you can use filterXPath instead).');
+        if (!class_exists(CssSelectorConverter::class)) {
+            throw new \RuntimeException('To filter with a CSS selector, install the CssSelector component ("composer require symfony/css-selector"). Or use filterXpath instead.');
         }
 
         $converter = new CssSelectorConverter($this->isHtml);