No global state for isolated tests and other fixes
Merge branch '2.3' into 2.5

* 2.3:
  No global state for isolated tests and other fixes
  [TwigBundle] Moved the setting of the default escaping strategy from the Twig engine to the Twig environment
  [Debug] fix checkip6
  [HttpFoundation] fixed error when an IP in the X-Forwarded-For HTTP header contains a port
  Update the note about origins of the CssSelector component.
  Use the correct cssselect library name in docblocks.