Fix RequestPartServletServerHttpRequest encoding issue

When using Appache Commons FileUpload, multi parts with binary data
(i.e. that are not actual files) are saved and then accessed as
String request parameters.

Before this change however the RequestPartServletServerHttpRequest
used a fixed encoding (UTF-8) while the parsing code in
CommonsFileUploadSupport/Resolver used the encoding from the
content-type header, or the request, or the FileUpload component.

This change does a best effort to determine the encoding of the
request parameter using a similar algorithm as the parsing side
that should work the same unless the encoding comes from the
FileUpload component which is not accessible.

Issue: SPR-13096

spring-web/src/main/java/org/springframework/web/multipart/support/RequestPartServletServerHttpRequest.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2012 the original author or authors.
+ * Copyright 2002-2015 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.
@@ -19,9 +19,11 @@ package org.springframework.web.multipart.support;
 import java.io.ByteArrayInputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.nio.charset.Charset;
 import javax.servlet.http.HttpServletRequest;
 
 import org.springframework.http.HttpHeaders;
+import org.springframework.http.MediaType;
 import org.springframework.http.server.ServerHttpRequest;
 import org.springframework.http.server.ServletServerHttpRequest;
 import org.springframework.util.ClassUtils;
@@ -111,9 +113,21 @@ public class RequestPartServletServerHttpRequest extends ServletServerHttpReques
 			}
 			else {
 				String paramValue = this.multipartRequest.getParameter(this.partName);
-				return new ByteArrayInputStream(paramValue.getBytes(FORM_CHARSET));
+				return new ByteArrayInputStream(paramValue.getBytes(determineEncoding()));
 			}
 		}
 	}
 
+	private String determineEncoding() {
+		MediaType contentType = getHeaders().getContentType();
+		if (contentType != null) {
+			Charset charset = contentType.getCharSet();
+			if (charset != null) {
+				return charset.name();
+			}
+		}
+		String encoding = this.multipartRequest.getCharacterEncoding();
+		return (encoding != null ? encoding : FORM_CHARSET);
+	}
+
 }