json 2.0 bug: comment nodes should be ignored by json:serialize

Comment nodes should be ignored by json:serialize function: please see failing tests:
- test/rbkt/zorba/json/json-snelson-serialize-object-03
- test/rbkt/zorba/json/json-snelson-serialize-array-12

For instance, test json-snelson-serialize-object-03:

import module namespace json = "http://www.zorba-xquery.com/modules/converters/json";

let $json :=
  <json type="object">
  <!--comment-->
    <pair name="a" type="number">1</pair>
  <!--comment-->
  </json>
return json:serialize( $json )

returns
{,"a":1,}
instead of
{"a":1}