Avoid async dispatch if completed in AsyncServerResponse

This commit checks whether the CompletableFuture<ServerResponse> passed
to AsyncServerResponse.async has been completed, and if so returns a
CompletedAsyncServerResponse that simply delegates to the completed
response, instead of the DefaultAsyncServerResponse that uses async
dispatch.

Closes gh-32223

spring-webmvc/src/test/java/org/springframework/web/servlet/function/DefaultAsyncServerResponseTests.java

@@ -28,7 +28,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 class DefaultAsyncServerResponseTests {
 
 	@Test
-	void block() {
+	void blockCompleted() {
 		ServerResponse wrappee = ServerResponse.ok().build();
 		CompletableFuture<ServerResponse> future = CompletableFuture.completedFuture(wrappee);
 		AsyncServerResponse response = AsyncServerResponse.create(future);
@@ -36,4 +36,21 @@ class DefaultAsyncServerResponseTests {
 		assertThat(response.block()).isSameAs(wrappee);
 	}
 
+	@Test
+	void blockNotCompleted() {
+		ServerResponse wrappee = ServerResponse.ok().build();
+		CompletableFuture<ServerResponse> future = CompletableFuture.supplyAsync(() -> {
+			try {
+				Thread.sleep(500);
+				return wrappee;
+			}
+			catch (InterruptedException ex) {
+				throw new RuntimeException(ex);
+			}
+		});
+		AsyncServerResponse response = AsyncServerResponse.create(future);
+
+		assertThat(response.block()).isSameAs(wrappee);
+	}
+
 }