Fix behavour for numbers

[alexander-schranz]

Fixes https://github.com/apostrophecms/sanitize-html/issues/593

Summary

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2. npm run test

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• [x] Bug fix
• [ ] New feature
• [ ] Refactor
• [ ] Documentation
• [ ] Build-related changes
• [ ] Other

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• [x] It includes a) the existing issue ID being resolved, b) a convincing reason for adding this feature, or c) a clear description of the bug it resolves
• [x] The changelog is updated
• [-] Related documentation has been updated
• [x] Related tests have been updated

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[alexander-schranz]

@boutell Thank you for the fast responses and review 👍