String getCertificate() {
    return configuration.get(CERTIFICATE).orElseThrow(() -> new IllegalArgumentException("Identity provider certificate is missing"));
  }
@Test
  public void fail_to_get_certificate_when_null() {
    assertThatThrownBy(() -> underTest.getCertificate())
      .isInstanceOf(IllegalArgumentException.class)
      .hasMessage("Identity provider certificate is missing");
  }