Merge branch '3.4'

* 3.4:
  [HttpFoundation] Add Request::HEADER_X_FORWARDED_AWS_ELB const
  [DI] Avoid private call to Container::has()
  Fixing missing abstract attribute in XmlDumper
  [Form] Remove DateTimeToStringTransformer $parseUsingPipe option
  [Yaml] parse PHP constants in mapping keys
  don't call getTrustedHeaderName() if possible
  Fix file perms
  Fixed filename in help text for update-data.php