Include quoted printable filename in ContentDisposition::toString

This commit ensures the ContentDisposition class prints the filename in
both in the regular filename parameter and the extended filename*
parameter (RFC 5987).
Quoted printable (RFC 2047) is used to encode any non-ASCII characters
in the regular filename parameter.

Closes gh-29861

spring-web/src/test/java/org/springframework/http/ContentDispositionTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2022 the original author or authors.
+ * Copyright 2002-2023 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.
@@ -249,7 +249,9 @@ class ContentDispositionTests {
 						.name("name")
 						.filename("中文.txt", StandardCharsets.UTF_8)
 						.build().toString())
-				.isEqualTo("form-data; name=\"name\"; filename*=UTF-8''%E4%B8%AD%E6%96%87.txt");
+				.isEqualTo("form-data; name=\"name\"; " +
+						"filename=\"=?UTF-8?Q?=E4=B8=AD=E6=96=87.txt?=\"; " +
+						"filename*=UTF-8''%E4%B8%AD%E6%96%87.txt");
 	}
 
 	@Test