GridView CheckboxColumn checkboxOptions won't accept closure

[HenryVolkmer]

Prerequisites

• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• The issue still exists against the latest master branch of yii2-grid.
• This is not an usage question. I confirm having gone through and read the documentation and demos.
• This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• I have attempted to find the simplest possible steps to reproduce the issue.
• [] I have included a failing test as a pull request (Optional).

Steps to reproduce the issue

1. Define kartik\grid\CheckboxColumn as Grid Col
2. configure the checkboxOptions property as Closure as documented in yii\grid\CheckboxColumn:

function ($model, $key, $index, $column) {
   return ['checked' => 'checked'];
}

Expected behavior and actual behavior

expected is:
all Checkboxes are rendered with the html-attribute 'checked="checked"'.

Environment

Ubuntu LAMP PHP 7.2

Browsers

• Google Chrome
• Mozilla Firefox
• Internet Explorer
• Safari

Operating System

• Windows
• Mac OS X
• [X ] Linux
• Mobile

Libraries

• jQuery version:
• yii2-grid version: master

Isolating the problem

The CheckboxColumn.php treat the checkboxOptions property as an array.
PR assigned.