Improve WebFlux Protobuf support

- Update javadoc for decoding default instances
 - Refactor and simplify tests
 - Add missing tests
 - Refactor decoding with flatMapIterable instead of
   concatMap and avoid recursive call

Issue: SPR-15776

spring-webflux/src/test/java/org/springframework/web/reactive/result/method/annotation/ProtobufIntegrationTests.java

@@ -129,6 +129,17 @@ public class ProtobufIntegrationTests extends AbstractRequestMappingIntegrationT
 				.verifyComplete();
 	}
 
+	@Test
+	public void defaultInstance() {
+		Mono<Msg> result = this.webClient.get()
+				.uri("/default-instance")
+				.retrieve()
+				.bodyToMono(Msg.class);
+
+		StepVerifier.create(result)
+				.verifyComplete();
+	}
+
 	@RestController
 	@SuppressWarnings("unused")
 	static class ProtobufController {
@@ -153,6 +164,11 @@ public class ProtobufIntegrationTests extends AbstractRequestMappingIntegrationT
 			return Mono.empty();
 		}
 
+		@GetMapping("default-instance")
+		Mono<Msg> defaultInstance() {
+			return Mono.just(Msg.getDefaultInstance());
+		}
+
 	}
 
 	@Configuration