Xlsx Style Writer Minor Refactoring

[oleibman]

In our unit tests, we can test the Worksheet Xml which PhpSpreadsheet will generate without having to actually write the spreadsheet to and read it from disk. This refactoring allows us to do the same for Styles, as demonstrated in the new StylesWriterTest.

This is:

• a bugfix
• a new feature
• refactoring
• additional unit tests

Checklist:

• Changes are covered by unit tests
  • Changes are covered by existing unit tests
  • New unit tests have been added
• 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 and a link to the pull request if applicable
• Documentation is updated as necessary