Make 'Content-Disposition' header case insensitive

Previously 'Content-Disposition' was passed to Part.getHeader(String).
However the Javadoc for that method specifies the header should be
case insensitive. Note that the JavaDoc in tomcat-servlet-api doesn't
mention this. It can only be found in the official api JavaDoc:

http://download.oracle.com/otndocs/jcp/servlet-3.0-fr-oth-JSpec/

Issue: SPR-9149

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

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2011 the original author or authors.
+ * Copyright 2002-2012 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.
@@ -41,7 +41,7 @@ import org.springframework.web.multipart.MultipartFile;
  */
 public class StandardMultipartHttpServletRequest extends AbstractMultipartHttpServletRequest {
 
-	private static final String CONTENT_DISPOSITION = "Content-Disposition";
+	private static final String CONTENT_DISPOSITION = "content-disposition";
 
 	private static final String FILENAME_KEY = "filename=";