method on Iterable to replace null elements #190

Hello,

I just implement #190.

I add some tests for this feature but I'm not really sure of what it's testing (for example, tests were passing before implementation of this feature in the js runtime)

As I'm quite new to git/github workflows and completely new to ceylon development, I think you should take some time to review this contribution in details.

Feel free to make critics (or just tips) about my work, I'm opened about that.
I you think that I miss something (or did something wrong), do not hesitate to tell/explain me what the problem is so that I can understand and fix it.

Thanks,
Loic