Merge pull request #2721 from thiagotalma/master

Add id in diff link
code cleanup.
Fixes #2691: Null parameters will not be included in the generated URLs by `UrlManager`
Fixes #2713: using relations defined in behavior may throw exception in lazy loading.
added unit tests for boolean attributes

fixes #1022
Update ActiveFormTest.php
fix letter
Fixed references in the guide
Merge pull request #2727 from Asetss/master

fix letter
Fixes #2734: `FileCache::keyPrefix` defaults to empty string now
Fixes #2735 

Adds DateTimeInterface support for Formatter
updated changelog
updated changelog
Merge pull request #2736 from ivokund/patch-1

Fixes #2735 - Formatter support for DateTimeInterface
Fixes #2737
fixes #2740
Merge pull request #2741 from tonydspaniard/2740-captcha-action-using-obsolete-method

Fixes #2740
fixes #2739
Merge pull request #2742 from tonydspaniard/2739-rest-createaction-bug

fixes #2739
doc fix [skip ci]