[BC break/RFC] Fix default cell type treatment as numeric for XLSX

[rtek]

SpreadsheetML defines cell type attribute t to indicate the cell type.
The attribute is optional and defaults to "n" for numeric. The XLSX
Reader incorrectly defaults the cell type to "stringy" for both
undefined t attribute and t="n".

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?

SpreadsheetML defines the default cell type as numeric, but the XLSX reader treats a missing cell type or cell type = numeric as a "stringy" value. This means that the reader is discarding the numeric type information from the XML. In most cases this is masked by the DefaultBinder which explicitly preg's for numeric strings and correct the reader's behavior.

The bugs / inconsistencies are:

1. Strings that pass the numeric preg are incorrectly casted to numeric
2. TheIValueBinder is always presented with string, even when the XML defines the type as numeric
3. A numeric cell that contains non-numeric information is not treated as an exception

This is probably a BC break as it seems like a fundamental issue with the Reader - I would expect there is lots of code relying on numeric strings being casted to a number despite the XML declaring them as strings.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs.
If this is still an issue for you, please try to help by debugging it further and sharing your results.
Thank you for your contributions.

[oleibman]

Superseded by PR #4139.