When a character is less than or equal to single byte size (0xff),
yet it takes more than 1 byte in the current encoding, the
case folding code incorrectly put it in bitset instead of code
range. As a result, for utf8 encoding, casefold works incorrectly
on characters in range \u0080 to \u00ff (latin1 supplement).

Before fix:

* `"\u00c2"` `[\u00e0-\u00e5]` returns false
* `"\u00c2"` `[\u00e2]` returns false
* `"\u00c2"` `\u00e2` returns true