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);
 		}