code: @Test public void cache() throws Exception {
    server.enqueue(new MockResponse().setBody("A").addHeader("ETag: v1"));
    server.enqueue(new MockResponse().setResponseCode(HttpURLConnection.HTTP_NOT_MODIFIED));
    server.play();
    client.setOkResponseCache(cache);
    onSuccess(new Request.Builder().url(server.getUrl("/")).build()).assertCode(200).assertBody("A");
    assertNull(server.takeRequest().getHeader("If-None-Match"));
    onSuccess(new Request.Builder().url(server.getUrl("/")).build()).assertCode(200).assertBody("A");
    assertEquals("v1",server.takeRequest().getHeader("If-None-Match"));
  }
  