code: public MailContent close() {
          if (!ends.isEmpty())
              throw new IllegalStateException(
                      "Pending some closing. Some end() missing.");
          html.nl().a("<br></body></html>");
          text.nl();
          return this;
      }
docs: Closes this MailContent. Checks about the closing tags throwing and
  {@link IllegalStateException} if there are any pending.
  <p>
  The HTML version gets the <em>body</em> and <em>html</em> closing tags
  and both receive a final new line.
  
  @return the changed mail content object