Increase test coverage, and modify handleEnd behavior to only see correct tokens.

Previously, handleEnd was called for any end tag, except ones that were obviously
spurious because there were no parent tags. Now, it is only called for end tags
that are "approved." If an injector operates on the end tag, we automatically
punt. There may be some optimizations that could be made to this procedure,
but for now it's much more consistent.

Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

tests/HTMLPurifier/Strategy/MakeWellFormed/SkipInjector.php

@@ -0,0 +1,10 @@
+<?php
+
+class HTMLPurifier_Strategy_MakeWellFormed_SkipInjector extends HTMLPurifier_Injector
+{
+    public $name = 'EndRewindInjector';
+    public $needed = array('span');
+    public function handleElement(&$token) {
+        $token = array(clone $token, clone $token);
+    }
+}