fix xlsx Cell datavalidations bug

[erelke]

This is:

- [ *] 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?

Cell datavalidations settings not readed from xlsx files. This fix this issue.

[MarkBaker]

Thanks for highlighting this: fix implemented with additional unit tests to verify that reader works correctly