Get style as array

[Erwane]

This is:

- [ ] a bug report
- [x] a feature request
- [ ] **not** a usage question (ask them on https://stackoverflow.com/questions/tagged/phpspreadsheet or https://gitter.im/PHPOffice/PhpSpreadsheet)

Same as #580, with an use case.
Get Style as array, for unit testing.

Tests can be very simple with a PhpOffice\PhpSpreadsheet\Style::toArray() method

Maybe a method getAsArray(string $styleType) where styleType can be borders fill ...

Thanks

[PowerKiKi]

Duplicates #580