* fix#15798 (yii\grid\CheckboxColumn, yii\grid\RadioButtonColumn content option has no effect)
* fix#15798 update pl, call parent method
* add test for content options in checkboxcolumn and radiobuttoncolumn
* update changelog by #15798
* Added the RadioButtonColumn class and updated the changelog.
* - several enhancements:
- unit tests added
- changelog fixed
* Update CHANGELOG.md
* small markdown fixes
* fix travis: custom ID