Update DefaultValueValidator.php

Allow the default value to be a closure
Update CHANGELOG.md
Update DefaultValueValidator.php
Check if property is set

```getValue``` didn't check if the property on an object exists so wouldn't return the default value.
fix #3478

it correct work with integer keys of list
Method to unlink all relationship many to many in current model
add change to CHANGELOG,and mini refactoring
add #3520 to changelog
change nik name (case sensitive)
Add support all type relation
Fixes #3542: Removed requirement to specify `extensions` in application config
Adjustments according to comments on #3575
Fixed asset controller tests
added more types to pgsql schema, better detection

fixes #3578
fix #3501 replace . when generating input ids
fixes #3473, allow precision in pgsql abstract timestamp
fixed argument display in errorHandler
Merge branch 'master'

Conflicts:
	framework/CHANGELOG.md
removed `@link` tags from apidoc and replaced with markdown
Adding rotateByCopy to yii\log\FileTarget #3562