code: public Body build() {
          if (!ends.isEmpty())
              throw new IllegalStateException(
                  "Pending some closing. Some end() missing. ends=" + ends);
          html.a(POST_HTML);
          return new Body(html, text);
      }
docs: Ends this builder and returns a Body. 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 mail content object
  @throws IllegalStateException if there are pending tags to close