Merge branch '2.6' into 2.7

* 2.6:
  [Security] fix check for empty usernames
  [Form] updated exception message of ButtonBuilder::setRequestHandler()
  [travis] Fix deps=high jobs
  [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

Author: nicolas-grekas

ButtonBuilder.php

@@ -498,7 +498,7 @@ public function setMethod($method)
      */
     public function setRequestHandler(RequestHandlerInterface $requestHandler)
     {
-        throw new BadMethodCallException('Buttons do not support form processors.');
+        throw new BadMethodCallException('Buttons do not support request handlers.');
     }
 
     /**