docs: private String writeUpdateString(Map<String, String> param) {
          StringBuffer fieldStringBuffer = new StringBuffer();
          for(Entry<String, String> entry : param.entrySet()){
          	fieldStringBuffer.append("<field name=\"").append(entry.getKey()).append("\">")
          			.append(StringEscapeUtils.escapeXml(entry.getValue())).append("</field>");
          }
          fieldStringBuffer.insert(0,"<add><doc>").append("</doc></add>");
          
          return fieldStringBuffer.toString();
      }