Added delta for Request comparison

Sometimes, tests are failing because REQUEST_TIME
changes between the creation of the various sub-requests.

By using delta, we allow maximum of second of difference.

Example: https://travis-ci.org/symfony/symfony/jobs/17668158#L511
Merge branch '2.3' into 2.4

* 2.3:
  $default can be string
  Fix wording for Process class documentation
  Option can be bool too (eg. --force)
  [Form] Update DateTime objects only if the actual value has changed
  Revert "bug #10091 [Translation] Update PluralizationRules.php (guilhermeblanco)"
  [HttpFoundation] fixed typo
  Added delta for Request comparison
  add zh_TW validator translations
  Added Bulgarian translation for security component