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/main/java/org/springframework/test/web/reactive/server/HeaderAssertions.java

@@ -25,12 +25,15 @@ import org.springframework.http.HttpHeaders;
 import org.springframework.http.MediaType;
 import org.springframework.lang.Nullable;
 
-import static org.springframework.test.util.AssertionErrors.*;
+import static org.springframework.test.util.AssertionErrors.assertEquals;
+import static org.springframework.test.util.AssertionErrors.assertTrue;
+import static org.springframework.test.util.AssertionErrors.fail;
 
 /**
  * Assertions on headers of the response.
  *
  * @author Rossen Stoyanchev
+ * @author Brian Clozel
  * @since 5.0
  * @see WebTestClient.ResponseSpec#expectHeader()
  */
@@ -71,6 +74,17 @@ public class HeaderAssertions {
 		return this.responseSpec;
 	}
 
+	/**
+	 * Expect that the header with the given name is not present.
+	 */
+	public WebTestClient.ResponseSpec doesNotExist(String name) {
+		if (getHeaders().containsKey(name)) {
+			String message = getMessage(name) + " exists with value=[" + getHeaders().getFirst(name) + "]";
+			this.exchangeResult.assertWithDiagnostics(() -> fail(message));
+		}
+		return this.responseSpec;
+	}
+
 	/**
 	 * Expect a "Cache-Control" header with the given value.
 	 */
@@ -99,6 +113,32 @@ public class HeaderAssertions {
 		return assertHeader("Content-Type", mediaType, getHeaders().getContentType());
 	}
 
+	/**
+	 * Expect a "Content-Type" header with the given value.
+	 */
+	public WebTestClient.ResponseSpec contentType(String mediaType) {
+		return contentType(MediaType.parseMediaType(mediaType));
+	}
+
+	/**
+	 * Expect a "Content-Type" header compatible with the given value.
+	 */
+	public WebTestClient.ResponseSpec contentTypeCompatibleWith(MediaType mediaType) {
+		MediaType actual = getHeaders().getContentType();
+		String message = getMessage("Content-Type") + "=[" + actual.toString() + "]"
+				+ " is not compatible with [" + mediaType.toString() + "]";
+		this.exchangeResult.assertWithDiagnostics(() ->
+				assertTrue(message, actual.isCompatibleWith(mediaType)));
+		return this.responseSpec;
+	}
+
+	/**
+	 * Expect a "Content-Type" header compatible with the given value.
+	 */
+	public WebTestClient.ResponseSpec contentTypeCompatibleWith(String mediaType) {
+		return contentTypeCompatibleWith(MediaType.parseMediaType(mediaType));
+	}
+
 	/**
 	 * Expect an "Expires" header with the given value.
 	 */

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();