Add Flux<Part> ServerRequest.parts()

This commit introduces Flux<Part> ServerRequest.parts() that delegates
to ServerWebExchange.getParts() and offers an alternative,
streaming way of accessing multipart data.

Closes gh-23131

spring-test/src/main/java/org/springframework/mock/web/reactive/function/server/MockServerRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2018 the original author or authors.
+ * Copyright 2002-2019 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.
@@ -249,6 +249,13 @@ public final class MockServerRequest implements ServerRequest {
 		return (Mono<MultiValueMap<String, Part>>) this.body;
 	}
 
+	@Override
+	@SuppressWarnings("unchecked")
+	public Flux<Part> parts() {
+		Assert.state(this.body != null, "No body");
+		return (Flux<Part>) this.body;
+	}
+
 	@Override
 	public ServerWebExchange exchange() {
 		Assert.state(this.exchange != null, "No exchange");