Add doesExist() to HeaderAssertions for WebTestClient

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

@@ -32,6 +32,7 @@ import static org.springframework.test.util.AssertionErrors.*;
  *
  * @author Rossen Stoyanchev
  * @author Brian Clozel
+ * @author Sam Brannen
  * @since 5.0
  * @see WebTestClient.ResponseSpec#expectHeader()
  */
@@ -72,6 +73,18 @@ public class HeaderAssertions {
 		return this.responseSpec;
 	}
 
+	/**
+	 * Expect that the header with the given name is present.
+	 * @since 5.0.3
+	 */
+	public WebTestClient.ResponseSpec doesExist(String name) {
+		if (!getHeaders().containsKey(name)) {
+			String message = getMessage(name) + " does not exist";
+			this.exchangeResult.assertWithDiagnostics(() -> fail(message));
+		}
+		return this.responseSpec;
+	}
+
 	/**
 	 * Expect that the header with the given name is not present.
 	 */

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -37,6 +37,7 @@ import static org.mockito.Mockito.*;
  * Unit tests for {@link HeaderAssertions}.
  *
  * @author Rossen Stoyanchev
+ * @author Sam Brannen
  * @since 5.0
  */
 public class HeaderAssertionTests {
@@ -124,6 +125,26 @@ public class HeaderAssertionTests {
 		}
 	}
 
+	@Test
+	public void doesExist() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.APPLICATION_JSON_UTF8);
+		HeaderAssertions assertions = headerAssertions(headers);
+
+		// Success
+		assertions.doesExist("Content-Type");
+
+		try {
+			assertions.doesExist("Framework");
+			fail("Header should not exist");
+		}
+		catch (AssertionError error) {
+			Throwable cause = error.getCause();
+			assertNotNull(cause);
+			assertEquals("Response header 'Framework' does not exist", cause.getMessage());
+		}
+	}
+
 	@Test
 	public void doesNotExist() {
 		HttpHeaders headers = new HttpHeaders();