bugfix: char class casefold for certain chars

[haozhun]

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