Remove buffering of encoded parts in MultipartHttpMessageWriter

Closes gh-23518

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

@@ -27,7 +27,6 @@ import java.util.Map;
 import java.util.Optional;
 import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.function.Supplier;
-import java.util.stream.Collectors;
 
 import org.reactivestreams.Publisher;
 import reactor.core.publisher.Flux;
@@ -249,8 +248,8 @@ public class MultipartHttpMessageWriter extends LoggingCodecSupport
 	private Flux<DataBuffer> encodePartValues(
 			byte[] boundary, String name, List<?> values, DataBufferFactory bufferFactory) {
 
-		return Flux.concat(values.stream().map(v ->
-				encodePart(boundary, name, v, bufferFactory)).collect(Collectors.toList()));
+		return Flux.fromIterable(values)
+				.concatMap(value -> encodePart(boundary, name, value, bufferFactory));
 	}
 
 	@SuppressWarnings("unchecked")

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -409,12 +409,13 @@ public class WebClientIntegrationTests {
 		byte[] expected = Files.readAllBytes(resource.getFile().toPath());
 		Flux<DataBuffer> body = DataBufferUtils.read(resource, new DefaultDataBufferFactory(), 4096);
 
-		this.webClient.post()
+		Mono<Void> result = this.webClient.post()
 				.uri("/")
 				.body(body, DataBuffer.class)
 				.retrieve()
-				.bodyToMono(Void.class)
-				.block(Duration.ofSeconds(5));
+				.bodyToMono(Void.class);
+
+		StepVerifier.create(result).verifyComplete();
 
 		expectRequest(request -> {
 			ByteArrayOutputStream actual = new ByteArrayOutputStream();