Fixes h() to not return empty string when input string contains invalid code unit sequence.

Caught me by surprise when I passed an object containing some invalid code unit sequence (binary data) to `debug()` but it returned me an empty string, even though there are plaintext properties (which I was interested in).

Even for normal usage, I think `h()` should not return a result that's far too different from the original input, especially when there are other valid plantext characters.