Fix for Xls writer wrong cells and sheet selection.

[jimmy4o4]

This is:

- [x] a bugfix
- [ ] a new feature

Checklist:

• Changes are covered by unit tests
• Code style is respected
• Commit message explains why the change is made (see https://github.com/erlang/otp/wiki/Writing-good-commit-messages)
• CHANGELOG.md contains a short summary of the change
• Documentation is updated as necessary

Why this change is needed?

See #1250 .

[PowerKiKi]

This is a nice improvement, but we would need to cover it with testing. You can have a look at \PhpOffice\PhpSpreadsheetTests\Functional\AbstractFunctional and sibling classes to see how to test writing and reading files.

[jimmy4o4]

Fix for Xls BIFF window2 structure was needed too.

I've added some tests too.

All of this covers only problems with Xls. Ods and Xlsx are still affected, I'm not sure, if there is any possible solution for these file types.