apollo-portal/src/main/java/com/ctrip/framework/apollo/portal/spi/defaultimpl/DefaultEmailService.java/HTMLDataSource/getInputStream
class HTMLDataSource:    @Override
    public InputStream getInputStream() throws IOException {
      if (html == null) {
        throw new IOException("html message is null!");
      }
      return new ByteArrayInputStream(html.getBytes());
    }