Update hidden tag to support being disabled per WW-2566. Thanks to Jelmer Kuperus for the patch.

I added a unit test as well.

Files:
added: /struts/struts2/trunk/core/src/test/resources/org/apache/struts2/views/jsp/ui/Hidden-2.txt (try)
modified: /struts/struts2/trunk/core/src/site/resources/tags/hidden.html (try)
modified: /struts/struts2/trunk/core/src/test/java/org/apache/struts2/views/jsp/ui/HiddenTest.java (try)
modified: /struts/struts2/trunk/core/src/main/resources/template/simple/hidden.ftl (try)