fixes #2957

fix for https://github.com/yiisoft/yii2/pull/2958

See this for more details: https://github.com/yiisoft/yii/pull/3300

Refactored ActiveRecord::find() so that find(null) will also return an ActiveRecord instance instead of ActiveQuery.

Fixed phpdoc, removed unit tests for Formatter asRelativeTime that were relative to current time since it was impossible to specify correct expected values

fixes #1946