STR-2740: Support XHTML versions

Files:
modified: /struts/struts1/trunk/taglib/src/main/java/org/apache/struts/taglib/TagUtils.java (try)
modified: /struts/struts1/trunk/taglib/src/main/java/org/apache/struts/taglib/html/HtmlTag.java (try)
modified: /struts/struts1/trunk/taglib/src/main/resources/META-INF/tld/struts-html.tld (try)