WW-3824 changes encoding definition to uppercase for Ajax requests to match specification

git-svn-id: https://svn.apache.org/repos/asf/struts/struts2/trunk@1344134 13f79535-47bb-0310-9956-ffa450edef68

core/src/main/java/org/apache/struts2/dispatcher/Dispatcher.java

@@ -682,7 +682,7 @@ public class Dispatcher {
         }
         // check for Ajax request to use UTF-8 encoding strictly http://www.w3.org/TR/XMLHttpRequest/#the-send-method
         if ("XMLHttpRequest".equals(request.getHeader("X-Requested-With"))) {
-            encoding = "utf-8";
+            encoding = "UTF-8";
         }
 
         Locale locale = null;