Fix ClassCastException for JDK 9+

AuthenticationPrincipalArgumentResolverTests failed in JDK 9+ due to
its improved generic support and a ClassCastException.

Issue gh-7363

Author: rwinch

messaging/src/test/java/org/springframework/security/messaging/handler/invocation/reactive/AuthenticationPrincipalArgumentResolverTests.java

@@ -83,7 +83,7 @@ private void currentUserOnMonoUserDetails(@CurrentUser Mono<UserDetails> user) {
 	@Test
 	public void resolveArgumentWhenExpressionThenFound() {
 		Authentication authentication = TestAuthentication.authenticatedUser();
-		Mono<UserDetails> result = (Mono<UserDetails>) this.resolver.resolveArgument(arg0("authenticationPrincipalExpression"), null)
+		Mono<String> result = (Mono<String>) this.resolver.resolveArgument(arg0("authenticationPrincipalExpression"), null)
 				.subscriberContext(ReactiveSecurityContextHolder.withAuthentication(authentication))
 				.block();
 		assertThat(result.block()).isEqualTo(authentication.getName());