Fix for #2224

[lucasnetau]

Fixes #2224 Fix reading data validation flags allowBlank, showDropDown, showInputMessage, showErrorMessage in XLSX reader (was a loose comparison of SimpleXML object to integer), flag values may also be string true/false not just 0/1.

filter_var( $flag, FILTER_VALIDATE_BOOLEAN) chosen to handle both 0/1 and true/false values in the file being read.

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?

Data validation flags are lost when reading an XLSX file containing true/false boolean values and writing it out after editing.

[oleibman]

Are you able to add some unit tests to show that files which do have true/false rather than 1/0 are handled correctly?

[lucasnetau]

OK, I shall attempt a unit test for this PR.

The true/false values are in xlsx documents created or edited in LibreOffice. This is valid under the schema definition https://docs.microsoft.com/en-us/dotnet/api/documentformat.openxml.spreadsheet.datavalidation?view=openxml-2.8.1

The possible values for this attribute are defined by the W3C XML Schema boolean datatype.

And based on https://www.w3.org/TR/xmlschema-2/#boolean

An instance of a datatype that is defined as ·boolean· can have the following legal literals {true, false, 1, 0}.

[lucasnetau]

Unit tests added with both DataValidation defined with true/false and also 1/0

I've reverted the change to showDropDown as upon further investigation this flag is misdefined in the specification and acts as a hideDropDown, which is the reason behind the inversion in both Reader and Writer. I have instead added a brief comment on the relevant lines when it is read and written.

[oleibman]

Thank you for your contribution, and for the excellent documentation of what you've done.