Merge branch '2.7' into 2.8

* 2.7:
  [HttpKernel] Fix lowest dep
  [Security] fix check for empty usernames
  [Form] updated exception message of ButtonBuilder::setRequestHandler()
  [travis] Fix deps=high jobs
  Fix typo 'assets.package' => 'assets.packages' in UPGRADE-2.7
  [Serializer] Simplify AbstractNormalizer::prepareForDenormalization()
  [HttpFoundation] [PSR-7] Allow to use resources as content body and to return resources from string content
  [DependencyInjection] Remove unused code in XmlFileLoader
  [HttpFoundation] Behaviour change in PHP7 for substr
  bumped Symfony version to 2.3.32
  updated VERSION for 2.3.31
  update CONTRIBUTORS for 2.3.31
  updated CHANGELOG for 2.3.31

Conflicts:
	src/Symfony/Bridge/Twig/composer.json
	src/Symfony/Bundle/FrameworkBundle/composer.json
	src/Symfony/Component/HttpKernel/composer.json

Author: nicolas-grekas

Normalizer/AbstractNormalizer.php

@@ -272,19 +272,7 @@ protected function getAllowedAttributes($classOrObject, array $context, $attribu
      */
     protected function prepareForDenormalization($data)
     {
-        if (is_array($data) || is_object($data) && $data instanceof \ArrayAccess) {
-            $normalizedData = $data;
-        } elseif (is_object($data)) {
-            $normalizedData = array();
-
-            foreach ($data as $attribute => $value) {
-                $normalizedData[$attribute] = $value;
-            }
-        } else {
-            $normalizedData = array();
-        }
-
-        return $normalizedData;
+        return (array) $data;
     }
 
     /**