private final XMLWriter w(Number v) {
        write(v.toString());
        return this;
    }
Writes the given string. If markup is us ascii (default), and the wrapped writer encodes in UTF, this will write
the string <b>as is, with no transcoding</b> (for speed). Hence, this should never be used for just any content.

@return this for consistency
vespajlib/src/main/java/com/yahoo/text/XMLWriter.java