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 SHA: 8e81d50 passes.
Change test to support Ruby 1.9.3 on TravisCI.

- Ruby 2.0 returns an array for String#chars, but Ruby 1.9.3 returns an
  enumerator instead.
- Using String#split is less elegant, but allows the test to pass on
  TravisCI with either MRI interpreter.