[Yaml] fix for a HHVM bug
Merge branch '2.3' into 2.4

* 2.3:
  [Process] Fix #9160 : escaping an argument with a trailing backslash on windows fails
  [Process] Fix some unit tests that create the process object instead of delegate it to the implementation
  [Process] Make process tests more accurate on exception messages
  [Process] Fix process status in TTY mode
  [Process] Use assertSame instead of assertEquals to avoid comparison against `null`
  [HttpFoundation] added some unit tests
  10158 get vary multiple
  made Cookie stringification more robust
  [Yaml] fix for a HHVM bug