Support @ModelAttribute(binding=false) with WebFlux

Prior to this commit, @ModelAttribute(binding=false) was honored with
Spring Web MVC but not with WebFlux.

This commit adds support for disabling binding via @ModelAttribute with
WebFlux by adding a check to resolveArgument(...) in
ModelAttributeMethodArgumentResolver.

Closes spring-projectsgh-26856

spring-webflux/src/main/java/org/springframework/web/reactive/result/method/annotation/ModelAttributeMethodArgumentResolver.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -65,6 +65,7 @@
  *
  * @author Rossen Stoyanchev
  * @author Juergen Hoeller
+ * @author Sam Brannen
  * @since 5.0
  */
 public class ModelAttributeMethodArgumentResolver extends HandlerMethodArgumentResolverSupport {
@@ -122,7 +123,7 @@ public Mono<Object> resolveArgument(
 
 		return valueMono.flatMap(value -> {
 			WebExchangeDataBinder binder = context.createDataBinder(exchange, value, name);
-			return bindRequestParameters(binder, exchange)
+			return (bindingDisabled(parameter) ? Mono.empty() : bindRequestParameters(binder, exchange))
 					.doOnError(bindingResultMono::onError)
 					.doOnSuccess(aVoid -> {
 						validateIfApplicable(binder, parameter);
@@ -147,6 +148,16 @@ public Mono<Object> resolveArgument(
 		});
 	}
 
+	/**
+	 * Determine if binding should be disabled for the supplied {@link MethodParameter},
+	 * based on the {@link ModelAttribute#binding} annotation attribute.
+	 * @since 5.2.15
+	 */
+	private boolean bindingDisabled(MethodParameter parameter) {
+		ModelAttribute modelAttribute = parameter.getParameterAnnotation(ModelAttribute.class);
+		return (modelAttribute != null && !modelAttribute.binding());
+	}
+
 	/**
 	 * Extension point to bind the request to the target object.
 	 * @param binder the data binder instance to use for the binding