Closure for contentOptions not acceptable

[bscheshirwork]

Prerequisites

• [x] I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• [x] The issue still exists against the latest master branch of yii2-export.
• [x] This is not an usage question. I confirm having gone through and read the documentation and demos.
• [x] This is not a general programming / coding question. (Those should be directed to the webtips Q & A forum).
• [x] 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. use closure for 'contentOptions' of column

   
       'contentOptions' => function ($model) {
           return [
               'class' => 'text-break',
               'data-toggle' => 'tooltip',
               'data-placement' => 'top',
               'title' => someFunctionalForGenerateTitle($model),
           ];
       },

2. click download btn
3. have error in 
vendor/kartik-v/yii2-export/src/ExportMenu.php:1394

## Expected behavior and actual behavior

When I follow those steps, I see...
Error

I was expecting...
pick `class` from closure result

## Environment

#### Browsers

- [ ] Google Chrome
- [ ] Mozilla Firefox
- [ ] Internet Explorer
- [ ] Safari

#### Operating System

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

#### Libraries

- jQuery version:
- yii2-export version:

## Isolating the problem

- [ ] This bug happens [on the demos page](https://demos.krajee.com/export-demo)
- [x] The bug happens consistently across all tested browsers
- [x] This bug happens when using yii2-export without other plugins.

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[kartik-v]

Fixed via #329.