Add missing StatusResultMatcher for HTTP Status 451

Issue: SPR-13632

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

@@ -33,6 +33,7 @@ import static org.springframework.test.util.AssertionErrors.*;
  * @author Keesun Baik
  * @author Rossen Stoyanchev
  * @author Sebastien Deleuze
+ * @author Brian Clozel
  * @since 3.2
  */
 public class StatusResultMatchers {
@@ -561,6 +562,14 @@ public class StatusResultMatchers {
 		return matcher(HttpStatus.valueOf(431));
 	}
 
+	/**
+	 * Assert the response status code is {@code HttpStatus.UNAVAILABLE_FOR_LEGAL_REASONS} (451).
+	 * @since 4.3
+	 */
+	public ResultMatcher isUnavailableForLegalReasons() {
+		return matcher(HttpStatus.valueOf(451));
+	}
+
 	/**
 	 * Assert the response status code is {@code HttpStatus.INTERNAL_SERVER_ERROR} (500).
 	 */