HTTP headers such as "Expires", "Last-Modified" all use date
strings like "Tue, 21 Jul 2015 10:00:00 GMT". Prior to this commit,
there was no way to match those header values, besides formatting dates
manually.
This commit introduces a new HeaderResultMatcher to test those date
headers using a long timestamp:
```
this.mockMvc.perform(get("/persons/1").header("If-Modified-Since", now))
.andExpect(status().isNotModified())
.andExpect(header().dateValue("Last-Modified", timestamp));
```
Issue: SPR-13263
cf2aed9d00