Only write non-default charset in MultipartWriterSupport

This commit only writes the 'charset' parameter in the written headers
if it is non-default (not UTF-8), since RFC7578 states that the only
allowed parameter is 'boundary'.

Closes gh-25885

Author: poutsma

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

@@ -149,7 +149,9 @@ public HttpMessageWriter<MultiValueMap<String, String>> getFormWriter() {
 	/**
 	 * Set the character set to use for part headers such as
 	 * "Content-Disposition" (and its filename parameter).
-	 * <p>By default this is set to "UTF-8".
+	 * <p>By default this is set to "UTF-8". If changed from this default,
+	 * the "Content-Type" header will have a "charset" parameter that specifies
+	 * the character set used.
 	 */
 	public void setCharset(Charset charset) {
 		Assert.notNull(charset, "Charset must not be null");

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

@@ -102,7 +102,11 @@ protected MediaType getMultipartMediaType(@Nullable MediaType mediaType, byte[]
 			params.putAll(mediaType.getParameters());
 		}
 		params.put("boundary", new String(boundary, StandardCharsets.US_ASCII));
-		params.put("charset", getCharset().name());
+		Charset charset = getCharset();
+		if (!charset.equals(StandardCharsets.UTF_8) &&
+				!charset.equals(StandardCharsets.US_ASCII) ) {
+			params.put("charset", getCharset().name());
+		}
 
 		mediaType = (mediaType != null ? mediaType : MediaType.MULTIPART_FORM_DATA);
 		mediaType = new MediaType(mediaType, params);

spring-web/src/test/java/org/springframework/http/codec/multipart/MultipartHttpMessageWriterTests.java

@@ -190,7 +190,7 @@ public void writeMultipartRelated() {
 		assertThat(contentType.isCompatibleWith(mediaType)).isTrue();
 		assertThat(contentType.getParameter("type")).isEqualTo("foo");
 		assertThat(contentType.getParameter("boundary")).isNotEmpty();
-		assertThat(contentType.getParameter("charset")).isEqualTo("UTF-8");
+		assertThat(contentType.getParameter("charset")).isNull();
 
 		MultiValueMap<String, Part> requestParts = parse(this.response, hints);
 		assertThat(requestParts.size()).isEqualTo(2);