Additional test cases for isogram

Few additional tests were added to `isogram` exercise in [canonical data](https://github.com/exercism/problem-specifications/blob/master/exercises/isogram/canonical-data.json):

* https://github.com/exercism/problem-specifications/pull/1307
* https://github.com/exercism/problem-specifications/pull/1307

These are recent few changes, but our tests might be lacking some more changes (I did not check). The task is to make the tests in this track in sync with the canonical data. Add [tests](https://github.com/exercism/reasonml/blob/master/exercises/isogram/__tests__/Isogram_test.re) and make changes to [example solution](https://github.com/exercism/reasonml/blob/master/exercises/isogram/src/Example.re) if required.