Eliminate TODO in Reader/Xml/Style/Border

[oleibman]

Also clean up the code a bit as long as I was in there.

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

[oleibman]

One of the problems with Coveralls. The only module which I changed was 100% covered before, and still is. But I reduced the number of relevant lines from 31 to 27, so overall coverage was consequently reduced by a minuscule amount. C'est la vie.