Do not convert big5 to unicode compatibility characters

[czchen]

Big5 has two characters that will convert to unicode compatibility characters due to duplicated encoding in Big5. Since Big5 <-> UTF8 conversions do not guarantee round-trip convertibility because of broken encoding, we shall not generate unicode compatibility characters.

Ref

• RFC 1922.

[czchen]

Unicode compatibility characters involved are u+fa0c, u+fa0d.