Merge branch '6.0' into 6.1

* 6.0:
  [String] Fix snake conversion
  [DowCrawler] Fix locale-sensitivity of whitespace normalization
  suggest to install the Twig bundle when the required component is already installed
  Update PULL_REQUEST_TEMPLATE.md
  [Serializer] Fix throwing right exception in ArrayDenormalizer with invalid type

Crawler.php

@@ -555,7 +555,7 @@ public function text(string $default = null, bool $normalizeWhitespace = true):
         $text = $this->getNode(0)->nodeValue;
 
         if ($normalizeWhitespace) {
-            return trim(preg_replace('/(?:\s{2,}+|[^\S ])/', ' ', $text));
+            return trim(preg_replace("/(?:[ \n\r\t\x0C]{2,}+|[\n\r\t\x0C])/", ' ', $text), " \n\r\t\x0C");
         }
 
         return $text;