Change header encoding to UTF8 in DefaultPartHttpMessageReader

This commit changes the encoding used to parse multipart headers from
ISO-8859-1 to UTF-8, in accordance with RFC 7578.

Closes gh-26736

spring-web/src/test/java/org/springframework/http/codec/multipart/DefaultPartHttpMessageReaderTests.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.
@@ -23,6 +23,7 @@ import java.lang.annotation.RetentionPolicy;
 import java.lang.annotation.Target;
 import java.nio.file.Files;
 import java.nio.file.Path;
+import java.util.Collections;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.TimeUnit;
 import java.util.stream.Stream;
@@ -251,6 +252,24 @@ public class DefaultPartHttpMessageReaderTests  {
 		latch.await();
 	}
 
+	@ParameterizedDefaultPartHttpMessageReaderTest
+	public void utf8Headers(String displayName, DefaultPartHttpMessageReader reader) throws InterruptedException {
+		MockServerHttpRequest request = createRequest(
+				new ClassPathResource("utf8.multipart", getClass()), "\"simple-boundary\"");
+
+		Flux<Part> result = reader.read(forClass(Part.class), request, emptyMap());
+
+		CountDownLatch latch = new CountDownLatch(1);
+		StepVerifier.create(result)
+				.consumeNextWith(part -> {
+					assertThat(part.headers()).containsEntry("Føø", Collections.singletonList("Bår"));
+					testPart(part, null, "This is plain ASCII text.", latch);
+				})
+				.verifyComplete();
+
+		latch.await();
+	}
+
 
 	private void testBrowser(DefaultPartHttpMessageReader reader, Resource resource, String boundary)
 			throws InterruptedException {