Add double-munging protection by checking if the domains are the same.

Previously, if an absolute munge URL location was used, HTML passed through
HTML Purifier multiple times would be munged multiple times. This patch
checks if the output URI has the same URI as the input URI; if they do,
the munge is considered unnecessary and discarded.

Requested-by: Chris <justbittin@gmail.com>
Signed-off-by: Edward Z. Yang <edwardzyang@thewritingpot.com>

tests/HTMLPurifier/URIFilter/MungeTest.php

@@ -112,4 +112,9 @@ class HTMLPurifier_URIFilter_MungeTest extends HTMLPurifier_URIFilterHarness
         $this->assertFiltering('http://google.com', '/links/http%3A%2F%2Fgoogle.com/0072e2f817fd2844825def74e54443debecf0892');
     }
     
+    function testMungeIgnoreSameDomain() {
+        $this->setMunge('http://example.com/%s');
+        $this->assertFiltering('http://example.com/foobar');
+    }
+    
 }