Part of https://github.com/behdad/fonttools/issues/236
Now we fallback to ASCII for unknown encodings. Not sure if this might be a bad idea.
The main user-visible difference is that if there's an ASCII-only text in an unknown
encoding, we still "decode" it and use unicode="True" instead of unicode="False".
Or is assuming that any unsupported encoding is ASCII-compatible too intrusive?