Refactor Maildir::Message#remove_flag.

 Refactor Maildir::Message#remove_flag.

- Close ktheory/maildir#10.
- Handle strings, arrays, and nil with equal grace.
- Handle empty strings and arrays gracefully.
- Continue to enforce arity of method.
- Update RDoc of method signature.
- Ensure test introduced in 8e81d50b07e904faf65ed9d447754c1c51bd2720 passes.