public SerializeWriter append(CharSequence csq) {
        String s = (csq == null ? "null" : csq.toString());
        write(s, 0, s.length());
        return this;
    }
Writes characters to the buffer.

@param c the data to be written
@param off the start offset in the data
@param len the number of chars that are written
src/main/java/com/alibaba/fastjson/serializer/SerializeWriter.java