Tests act as specifications, not documentation. They keep certain behaviour fixed. Of course it should be documented as well, if it is not obvious / the user needs know about it.

Personally, as stated in some of my earlier comments, I'm for explicity, so rather fail on everything that is not an ascii character and let the user specify what she wants if that's not enough. If the user specifies the wrong encoding and it still suceeds it's his problem. That's easy to define, test, document, understand, implement and hardly an edge case.

If we just assume a platform specific default encoding and it succeeds but is actually wrong, hey that wasn't so good after all. But I'm half-way okay with it, since it shouldn't really matter for this application, anyway...