Use Intl.js Polyfill as `Intl` mock in unit tests

Formatted output differs across implementations; e.g. IE11 adds bidi chars:
http://stackoverflow.com/questions/25574963/ies-tolocalestring-has-strange-characters-in-results

Therefore makes it difficult to unit test just these helpers. In order to overcome these problems, we can simply **force** the use of the Intl.js Polyfill as an `Intl` mock.