Add missing test

Author: sbrannen

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

@@ -105,6 +105,17 @@ void valueMatches() {
 						"[.*ISO-8859-1.*]"));
 	}
 
+	@Test
+	void valueMatchesWithNonexistentHeader() {
+		HttpHeaders headers = new HttpHeaders();
+		headers.setContentType(MediaType.parseMediaType("application/json;charset=UTF-8"));
+		HeaderAssertions assertions = headerAssertions(headers);
+
+		assertThatExceptionOfType(AssertionError.class)
+				.isThrownBy(() -> assertions.valueMatches("Content-XYZ", ".*ISO-8859-1.*"))
+				.withMessage("Response header 'Content-XYZ' not found");
+	}
+
 	@Test
 	void valuesMatch() {
 		HttpHeaders headers = new HttpHeaders();