XSS Vulnerability in HTML Writer

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? To prevent a potential XSS attack when loading a spreadsheet from an unknown source and saving as HTML