Merge branch '2.3' into 2.7

* 2.3:
  [HttpFoundation] Fix transient test
  [HttpFoundation] Add a dependency on the mbstring polyfill
  add readme files where missing
  Don't use reflections when possible
  [Form] Update form tests after the ICU data update
  [Intl] Update tests and the number formatter to match behaviour of the intl extension
  [Intl] Update the ICU data to version 55
  [Intl] Fix the update-data.php script in preparation for ICU 5.5
  Use constant instead of function call.
  fixed test name
  automatically generate safe fallback filename

Conflicts:
	src/Symfony/Component/Debug/Debug.php
	src/Symfony/Component/HttpFoundation/composer.json
	src/Symfony/Component/Serializer/Tests/Normalizer/GetSetMethodNormalizerTest.php

Author: nicolas-grekas

Debug.php

@@ -45,7 +45,7 @@ public static function enable($errorReportingLevel = null, $displayErrors = true
             error_reporting(-1);
         }
 
-        if ('cli' !== php_sapi_name()) {
+        if ('cli' !== PHP_SAPI) {
             ini_set('display_errors', 0);
             ExceptionHandler::register();
         } elseif ($displayErrors && (!ini_get('log_errors') || ini_get('error_log'))) {