Improve `exist` validator to support array attributes
Refactoring
Refactoring
ExistValidator count logic fix
CHANGELOG
`allowArray` introduced.
CHANGELOG updated
ExistValidator refactoring
ExistValidator bugfix
ExistValidator::validateValue() fixed
ExistValidator final refactoring. `allowArray` checking also added to `validateValue` method.
Merge PR #3406 branch 'exist-validator-enhancement' of https://github.com/creocoder/yii2

* 'exist-validator-enhancement' of https://github.com/creocoder/yii2:
  ExistValidator final refactoring. `allowArray` checking also added to `validateValue` method.
  New tests added
  New tests added
  ExistValidator::validateValue() fixed
  Tests excess dump removed
  ExistValidator bugfix
  Tests updated
  ExistValidator refactoring
  CHANGELOG updated
  `allowArray` introduced.
  CHANGELOG
  ExistValidator count logic fix
  Tests updated
  Tests updated
  Tests updated
  Refactoring
  Refactoring
  Improve `exist` validator to support array attributes

Conflicts:
	framework/CHANGELOG.md