@Test
public void shouldServeBuiltinDocument() throws Exception {

    AbstractHtmlErrorHandler handler
            = new SampleHtmlErrorHanlder("500", "MSG_TOKEN", "EXPLANATION_TOKEN", null);
    handler.serve(response);
    assertThat(response.getStatus(), containsString("500"));
    assertThat(outputStream.toString(), containsString("MSG_TOKEN"));
    assertThat(outputStream.toString(), containsString("EXPLANATION_TOKEN"));
}