Testing Spring HTTP Caching

I have springboot application that enable http caching. I'm using webRequest.checkModifiedSince as describe in here. When running my application in browser I get correct result, 200 status code on first hit and 304 on next hit. But when I run maven test to my application it seems that webRequest.checkModifiedSince always return false.

Here is my testcase :

@Test
public void checkCache() throws Exception {
    MvcResult res = this.mockMvc.perform(get("/resource/to/cache.jpg"))
                        .andExpect(status().isOk())
                        .andReturn();

    String date = res.getResponse().getHeader("Last-Modified");
    HttpHeaders headers = new HttpHeaders();
    headers.setIfModifiedSince(Long.parseLong(date));
    headers.setCacheControl("max-age=0");

    this.mockMvc.perform(get("same/resource/as/above.jpg")
                .headers(headers))
                .andExpect(status().isNotModified());
}

Did I do something wrong here?