Fix Xlsx Writer's handling of decimal commas

[makedin]

Perform the comma -> period substitution in Xlsx Writer writeCell()
before checking if the decimal value has a period in it. The previous
behavior led to decimals of the form "1,1" to become "1.1.0".

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 #1281

[MarkBaker]

Thanks you for this solution to the locale problem