Correctly detect Optional return type for @HttpExchange methods

Prior to this commit, a ClassCastException was thrown for an Optional
return type for an @HttpExchange method. This is because the check for
an Optional return type was based on the type contained in the Optional
instead of the Optional itself.

Closes gh-28493

spring-web/src/main/java/org/springframework/web/service/invoker/HttpServiceMethod.java

@@ -324,7 +324,7 @@ final class HttpServiceMethod {
 				responseFunction = initBodyFunction(client, actualParam, reactiveAdapter);
 			}
 
-			boolean blockForOptional = actualType.equals(Optional.class);
+			boolean blockForOptional = returnType.equals(Optional.class);
 			return new ResponseFunction(responseFunction, reactiveAdapter, blockForOptional, blockTimeout);
 		}
 

spring-web/src/test/java/org/springframework/web/service/invoker/HttpServiceMethodTests.java

@@ -16,6 +16,8 @@
 
 package org.springframework.web.service.invoker;
 
+import java.util.Optional;
+
 import io.reactivex.rxjava3.core.Completable;
 import io.reactivex.rxjava3.core.Flowable;
 import io.reactivex.rxjava3.core.Single;
@@ -131,6 +133,9 @@ public class HttpServiceMethodTests {
 		String body = service.getBody();
 		assertThat(body).isEqualTo("requestToBody");
 
+		Optional<String> optional = service.getBodyOptional();
+		assertThat(optional).isEqualTo(Optional.of("requestToBody"));
+
 		ResponseEntity<String> entity = service.getEntity();
 		assertThat(entity.getBody()).isEqualTo("requestToEntity");
 
@@ -252,6 +257,9 @@ public class HttpServiceMethodTests {
 		@GetExchange
 		String getBody();
 
+		@GetExchange
+		Optional<String> getBodyOptional();
+
 		@GetExchange
 		ResponseEntity<Void> getVoidEntity();