[1.6.1] Fix bug (== v. ===) that caused merged in attribute definitions to be messed up

- Make our modified class_exists() check to work in both PHP 4 and 5
(todo: we need some unit tests for ElementDef)

git-svn-id: http://htmlpurifier.org/svnroot/htmlpurifier/trunk@1023 48356398-32a2-884e-a903-53898d9a118a

library/HTMLPurifier/ElementDef.php

@@ -95,7 +95,7 @@ class HTMLPurifier_ElementDef
         
         // later keys takes precedence
         foreach($def->attr as $k => $v) {
-            if ($k == 0) {
+            if ($k === 0) {
                 // merge in the includes
                 // sorry, no way to override an include
                 foreach ($v as $v2) {