Support Custom Headers for Multipart Async Data

This commit makes sure there is no custom Content-Disposition header
before setting one automatically.

This commit also adds a headers(Consumer<HttpHeaders>) method, so that
one can user the nicer methods of HttpHeaders, as opposed to basic
strings.

Issue: SPR-16376

spring-web/src/main/java/org/springframework/http/client/MultipartBodyBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -19,6 +19,7 @@ package org.springframework.http.client;
 import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
+import java.util.function.Consumer;
 
 import org.reactivestreams.Publisher;
 
@@ -187,6 +188,13 @@ public final class MultipartBodyBuilder {
 		 * @see HttpHeaders#add(String, String)
 		 */
 		PartBuilder header(String headerName, String... headerValues);
+
+		/**
+		 * Manipulate the part's headers with the given consumer.
+		 * @param headersConsumer a function that consumes the {@code HttpHeaders}
+		 * @return this builder
+		 */
+		PartBuilder headers(Consumer<HttpHeaders> headersConsumer);
 	}
 
 
@@ -208,6 +216,13 @@ public final class MultipartBodyBuilder {
 			return this;
 		}
 
+		@Override
+		public PartBuilder headers(Consumer<HttpHeaders> headersConsumer) {
+			Assert.notNull(headersConsumer, "'headersConsumer' must not be null");
+			headersConsumer.accept(this.headers);
+			return this;
+		}
+
 		public HttpEntity<?> build() {
 			return new HttpEntity<>(this.body, this.headers);
 		}

spring-web/src/main/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriter.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2017 the original author or authors.
+ * Copyright 2002-2018 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.
@@ -229,12 +229,13 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 	@SuppressWarnings("unchecked")
 	private <T> Flux<DataBuffer> encodePart(byte[] boundary, String name, T value) {
 		MultipartHttpOutputMessage outputMessage = new MultipartHttpOutputMessage(this.bufferFactory, getCharset());
+		HttpHeaders outputHeaders = outputMessage.getHeaders();
 
 		T body;
 		ResolvableType resolvableType = null;
 		if (value instanceof HttpEntity) {
 			HttpEntity<T> httpEntity = (HttpEntity<T>) value;
-			outputMessage.getHeaders().putAll(httpEntity.getHeaders());
+			outputHeaders.putAll(httpEntity.getHeaders());
 			body = httpEntity.getBody();
 			Assert.state(body != null, "MultipartHttpMessageWriter only supports HttpEntity with body");
 
@@ -247,24 +248,24 @@ public class MultipartHttpMessageWriter implements HttpMessageWriter<MultiValueM
 		else {
 			body = value;
 		}
-
 		if (resolvableType == null) {
 			resolvableType = ResolvableType.forClass(body.getClass());
 		}
 
-		if (body instanceof Resource) {
-			outputMessage.getHeaders().setContentDispositionFormData(name, ((Resource) body).getFilename());
-		}
-		else if (Resource.class.equals(resolvableType.getRawClass())) {
-			body = (T) Mono.from((Publisher<?>) body).doOnNext(o -> {
-				outputMessage.getHeaders().setContentDispositionFormData(name, ((Resource) o).getFilename());
-			});
-		}
-		else {
-			outputMessage.getHeaders().setContentDispositionFormData(name, null);
+		if (!outputHeaders.containsKey(HttpHeaders.CONTENT_DISPOSITION)) {
+			if (body instanceof Resource) {
+				outputHeaders.setContentDispositionFormData(name, ((Resource) body).getFilename());
+			}
+			else if (Resource.class.equals(resolvableType.getRawClass())) {
+				body = (T) Mono.from((Publisher<?>) body).doOnNext(o -> outputHeaders
+						.setContentDispositionFormData(name, ((Resource) o).getFilename()));
+			}
+			else {
+				outputHeaders.setContentDispositionFormData(name, null);
+			}
 		}
 
-		MediaType contentType = outputMessage.getHeaders().getContentType();
+		MediaType contentType = outputHeaders.getContentType();
 
 		final ResolvableType finalBodyType = resolvableType;
 		Optional<HttpMessageWriter<?>> writer = this.partWriters.stream()