Xlsx Reader External Data Validations Flag Missing

[oleibman]

Fix #2677. This PR supersedes #2679, written by @technghiath, which lacks tests, and probably doesn't solve the problem entirely. The code causing the problem appears to be the last remnant in Xlsx Reader which calls children using a namespace prefix rather than a namespace. That is changed, and tests are added where the tag is unexpectedly missing, and also where it uses a non-standard namespace prefix.

This is:

- [x] a bugfix
- [ ] a new feature
- [ ] refactoring
- [ ] additional unit tests

Checklist:

• [x] Changes are covered by unit tests
  • [x] Changes are covered by existing unit tests
  • [x] New unit tests have been added
• [x] Code style is respected
• [x] 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?

Provide an explanation of why this change is needed, with links to any Issues (if appropriate). If this is a bugfix or a new feature, and there are no existing Issues, then please also create an issue that will make it easier to track progress with this PR.