Switch order of multipart Content-Type directives

Since SPR-15205, the `FormHttpMessageConverter` is adding a `charset`
directive to the `Content-Type` request header in order to help servers
understand which charset is being used to encode headers of each part.

As reported in SPR-17030 and others, some servers are not parsing
properly such header values and assume that `boundary` is the last
directive in the `Content-Type` header.

This commit reorders the charset information right before the boundary
declaration to get around those issues.

Issue: SPR-17030

Author: bclozel

spring-web/src/main/java/org/springframework/http/converter/FormHttpMessageConverter.java

@@ -25,7 +25,7 @@
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collections;
-import java.util.HashMap;
+import java.util.LinkedHashMap;
 import java.util.List;
 import java.util.Map;
 import javax.mail.internet.MimeUtility;
@@ -351,11 +351,11 @@ private void writeMultipart(final MultiValueMap<String, Object> parts,
 			HttpOutputMessage outputMessage) throws IOException {
 
 		final byte[] boundary = generateMultipartBoundary();
-		Map<String, String> parameters = new HashMap<>(2);
-		parameters.put("boundary", new String(boundary, "US-ASCII"));
+		Map<String, String> parameters = new LinkedHashMap<>(2);
 		if (!isFilenameCharsetSet()) {
 			parameters.put("charset", this.charset.name());
 		}
+		parameters.put("boundary", new String(boundary, "US-ASCII"));
 
 		MediaType contentType = new MediaType(MediaType.MULTIPART_FORM_DATA, parameters);
 		HttpHeaders headers = outputMessage.getHeaders();

spring-web/src/test/java/org/springframework/http/converter/FormHttpMessageConverterTests.java

@@ -30,6 +30,8 @@
 import org.apache.commons.fileupload.FileUpload;
 import org.apache.commons.fileupload.RequestContext;
 import org.apache.commons.fileupload.disk.DiskFileItemFactory;
+import org.hamcrest.CoreMatchers;
+import org.hamcrest.Matchers;
 import org.junit.Test;
 
 import org.springframework.core.io.ClassPathResource;
@@ -148,7 +150,8 @@ public String getFilename() {
 		this.converter.write(parts, new MediaType("multipart", "form-data", StandardCharsets.UTF_8), outputMessage);
 
 		final MediaType contentType = outputMessage.getHeaders().getContentType();
-		assertNotNull("No boundary found", contentType.getParameter("boundary"));
+		// SPR-17030
+		assertThat(contentType.getParameters().keySet(), Matchers.contains("charset", "boundary"));
 
 		// see if Commons FileUpload can read what we wrote
 		FileItemFactory fileItemFactory = new DiskFileItemFactory();