Using transChoice with es5-shim in MSIE8 fails

I've added a new test html page which loads the popular [es5-shim library](https://github.com/es-shims/es5-shim). I've also added two test cases that, on that new page, fail to return the right string when using `transChoice` in Microsoft Internet Explorer 8.

Iterating over `_parts` in `transChoice` using the `in` operator pushed the shim-ed methods into the `_standardRules` array. One of that methods itself would then be returned as plural position 0.

Testing this with `phantomjs` is useless as the (new) failing tests only occur in an ES3 browser like MSIE8.
I'm sure you don't like having non-automated unit tests, but I'm not so sure what to do about it.