Add doesExist assertion method to HeaderResultMatchers

spring-test/src/main/java/org/springframework/test/web/servlet/result/HeaderResultMatchers.java

@@ -99,6 +99,15 @@ public class HeaderResultMatchers {
 				!result.getResponse().containsHeader(name));
 	}
 
+	/**
+	 * Assert that the named response header does exist.
+	 * @since 4.3
+	 */
+	public ResultMatcher doesExist(final String name) {
+		return result -> assertTrue("Response should contain header '" + name + "'",
+									result.getResponse().containsHeader(name));
+	}
+
 	/**
 	 * Assert the primary value of the named response header as a {@code long}.
 	 * <p>The {@link ResultMatcher} returned by this method throws an