Fixing the issue #3292

This change fixes the issue #3292. The details were mentioned in the comments to the issue.
Update DefaultValueValidator.php

Allow the default value to be a closure
Update CHANGELOG.md
Update DefaultValueValidator.php
Update config.php
BaseVarDumper::export() added
Doc comments for `BaseVarDumper::export()` added
Issue #3232 added to CHANGELOG
Object handling at `BaseVarDumper::export()` reworked
Check if property is set

```getValue``` didn't check if the property on an object exists so wouldn't return the default value.
add Korean Language Translation
Update yii.php
Update config.php

add "ko" value for languages array
guide WIP [skip ci]
Advanced app's common tests codeception generated files gitignored
guide WIP [skip ci]
Moved yii\rest\Controller::checkAccess() to ActiveController.
guide WIP [skip ci]
Tests excess dump removed
improved debugger on handling unreadable files

issue #3494