Add doesNotExist match to HeaderResultMatchers

Prior to this commit, one could not test for the absence of
a specific HTTP header in a response.
Using header("X-Custom-Header", Matchers.nullValue()) would not
work because it tests for an empty value of an existing header.

This commit adds a doesNotExist method on the
HeaderResultMatcher.

Issue: SPR-10771

spring-test-mvc/src/test/java/org/springframework/test/web/servlet/samples/standalone/resultmatchers/HeaderAssertionTests.java

@@ -35,7 +35,7 @@ import static org.springframework.test.web.servlet.setup.MockMvcBuilders.*;
 
 /**
  * Examples of expectations on response header values.
- * 
+ *
  * @author Rossen Stoyanchev
  * @author Sam Brannen
  */
@@ -110,6 +110,22 @@ public class HeaderAssertionTests {
 		}
 	}
 
+	// SPR-10771
+
+	@Test
+	public void doesNotExist() throws Exception {
+		this.mockMvc.perform(get("/persons/1"))
+				.andExpect(header().doesNotExist("X-Custom-Header"));
+	}
+
+	// SPR-10771
+
+	@Test(expected = AssertionError.class)
+	public void doesNotExistFail() throws Exception {
+		this.mockMvc.perform(get("/persons/1"))
+				.andExpect(header().doesNotExist(LAST_MODIFIED));
+	}
+
 	@Test
 	public void stringWithIncorrectResponseHeaderValue() throws Exception {
 		long unexpected = currentTime + 1;