Merge branch '2.8' into 3.3

* 2.8:
  [HttpFoundation] Use the correct syntax for session gc based on Pdo driver
  Removed assertDateTimeEquals() methods.
  Revert "bug #24987 [Console] Fix global console flag when used in chain (Simperfit)"
  Revert "bug #25487 [Console] Fix a bug when passing a letter that could be an alias (Simperfit)"
  Disable CSP header on exception pages only in debug
  Fixed submitting disabled buttons
  Fixed Button::setParent() when already submitted
  Improve assertions
  Improve assertions
  SCA: get rid of repetitive calls
  allow null values for root nodes in YAML configs
  [VarDumper] Fix docblock
  Improve phpdoc to make it more explicit

Author: fabpot