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

README.md

@@ -0,0 +1,10 @@
+WebProfilerBundle
+=================
+
+Resources
+---------
+
+  * [Contributing](https://symfony.com/doc/current/contributing/index.html)
+  * [Report issues](https://github.com/symfony/symfony/issues) and
+    [send Pull Requests](https://github.com/symfony/symfony/pulls)
+    in the [main Symfony repository](https://github.com/symfony/symfony)