As written, the strftime module's `getSuffix` method (responsible for
returning a date's ordinal string) works correctly for all dates in a
month _except_ 11, 12, and 13.
The existing code would return a string based on the last digit of the
date. In American English, these three dates use the `th` suffix rather
than `st`, `nd`, or `rd`.
This refactor/bug fix draws inspiration from Rails' ActiveSupport
ordinal inflector:
https://github.com/rails/rails/blob/main/activesupport/lib/active_support/locale/en.rb
The Ruby code above is designed to work with any number, so the bug fix
to this library is simpler.
The test suite is also updated with additional examples to verify the
correctness of these changes.
