Styles from property exportMenuHeaderStyle in kartik\grid\DataColumn are not applied for the header row of table when exporting to Excel 2007+ via ExportMenu widget.

[orioledev]

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. My code is:

   
   use kartik\export\ExportMenu;

$gridColumns = [ 'id', [ 'class' => \kartik\grid\DataColumn::class, 'attribute' => 'name', 'exportMenuHeaderStyle' => [ 'fill' => [ 'color' => ['argb' => \PhpOffice\PhpSpreadsheet\Style\Color::COLOR_GREEN], ], ], ], 'description', ]; ?>

<?= ExportMenu::widget([ 'dataProvider' => $dataProvider, 'columns' => $gridColumns, 'fontAwesome' => true, 'target' => ExportMenu::TARGET_SELF, 'showConfirmAlert' => false, 'clearBuffers' => true, 'initProvider' => true, 'headerStyleOptions' => [ ExportMenu::FORMAT_EXCEL_X => [ 'fill' => [ 'color' => ['argb' => \PhpOffice\PhpSpreadsheet\Style\Color::COLOR_YELLOW], ], ], ], ]);

2. Load the page with ExportMenu widget.

## Expected behavior and actual behavior

When I follow those steps, I see:

![2018-11-14_10-43-21](https://user-images.githubusercontent.com/6672987/48468681-9f4a7800-e7fd-11e8-8d9b-4409f92933d5.png)

I was expecting:

![2018-11-14_10-45-41](https://user-images.githubusercontent.com/6672987/48468691-a70a1c80-e7fd-11e8-8db8-fd7686e418ef.png)

I did not find the usages of exportMenuHeaderStyle for header cells in the generateHeader() method.
Also, in the generateBox() method common widget styles headerStyleOptions applied for all cells of header row.

## Environment

#### Browsers

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

#### Operating System

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

#### Libraries

- jQuery version: v3.2.1
- yii2-export version: version 1.3.7

## 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.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.