HPCC-12349 JSON outputUnicode outputs extra characters

JSON outputUnicode and outputUtf8 are handling byte count vs character count inconsistently when calling appendJSONStringValue.

Signed-off-by: Anthony Fishbeck <anthony.fishbeck@lexisnexis.com>
can you simplify to

    s.append(chlen, ch);
    remaining -= (chlen-1);

?