code: public ServerBuilder contentPreview(int length, Charset defaultCharset) {
          return contentPreviewerFactory(ContentPreviewerFactory.ofText(length, defaultCharset));
      }
docs: Sets the {@link ContentPreviewerFactory} creating a {@link ContentPreviewer} which produces the preview
  with the maxmium {@code length} limit for a request and a response of this {@link Server}.
  The previewer is enabled only if the content type of a request/response meets
  any of the following cases.
  <ul>
  <li>when it matches {@code text/*} or {@code application/x-www-form-urlencoded}</li>
  <li>when its charset has been specified</li>
  <li>when its subtype is {@code "xml"} or {@code "json"}</li>
  <li>when its subtype ends with {@code "+xml"} or {@code "+json"}</li>
  </ul>
  @param length the maximum length of the preview.
  @param defaultCharset the default charset for a request/response with unspecified charset in
  {@code "content-type"} header.