code: @Nonnull
    public DigestAuthServerBuilder setOpaque (@Nonnull final String sOpaque)
    {
      if (!HttpStringHelper.isQuotedTextContent (sOpaque))
        throw new IllegalArgumentException ("opaque is invalid: " + sOpaque);
  
      m_sOpaque = sOpaque;
      return this;
    }
docs: A string of data, specified by the server, which should be returned by the
  client unchanged in the Authorization header of subsequent requests with
  URIs in the same protection space. It is recommended that this string be
  base64 or hexadecimal data.
  
  @param sOpaque
  The opaque value. May not be <code>null</code>.
  @return this