Add new WebTestClient header assertions

This commit adds new header assertions for `WebTestClient`.

`doesNotExist` tests that a given header is not present:

    .expectHeader().doesNotExist("Cache-Control");

`contentTypeCompatibleWith` tests for MediaType compatibility:

    .expectHeader().contentTypeCompatibleWith("text/*");

Issue: SPR-16285

spring-test/src/test/java/org/springframework/test/web/reactive/server/HeaderAssertionTests.java

@@ -124,6 +124,48 @@ public class HeaderAssertionTests {
 		}
 	}
 
+	@Test
+	public void doesNotExist() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_JSON_UTF8);
+		HeaderAssertions assertions = headerAssertions(headers);
+
+		// Success
+		assertions.doesNotExist("Framework");
+
+		try {
+			assertions.doesNotExist("Content-Type");
+			fail("Existing header expected");
+		}
+		catch (AssertionError error) {
+			Throwable cause = error.getCause();
+			assertNotNull(cause);
+			assertEquals("Response header 'Content-Type' exists with " +
+					"value=[application/json;charset=UTF-8]", cause.getMessage());
+		}
+	}
+
+	@Test
+	public void contentTypeCompatibleWith() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_XML);
+		HeaderAssertions assertions = headerAssertions(headers);
+
+		// Success
+		assertions.contentTypeCompatibleWith(MediaType.parseMediaType("application/*"));
+
+		try {
+			assertions.contentTypeCompatibleWith(MediaType.TEXT_XML);
+			fail("MediaTypes not compatible expected");
+		}
+		catch (AssertionError error) {
+			Throwable cause = error.getCause();
+			assertNotNull(cause);
+			assertEquals("Response header 'Content-Type'=[application/xml] " +
+					"is not compatible with [text/xml]", cause.getMessage());
+		}
+	}
+
 	@Test
 	public void cacheControl() {
 		CacheControl control = CacheControl.maxAge(1, TimeUnit.HOURS).noTransform();