Add support for proprietary "background" attribute in table elements.

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

tests/HTMLPurifier/AttrTransform/BackgroundTest.php

@@ -0,0 +1,39 @@
+<?php
+
+class HTMLPurifier_AttrTransform_BackgroundTest extends HTMLPurifier_AttrTransformHarness
+{
+    
+    function setUp() {
+        parent::setUp();
+        $this->obj = new HTMLPurifier_AttrTransform_Background();
+    }
+    
+    function testEmptyInput() {
+        $this->assertResult( array() );
+    }
+    
+    function testBasicTransform() {
+        $this->assertResult(
+            array('background' => 'logo.png'),
+            array('style' => 'background-image:url(logo.png);')
+        );
+    }
+    
+    function testPrependNewCSS() {
+        $this->assertResult(
+            array('background' => 'logo.png', 'style' => 'font-weight:bold'),
+            array('style' => 'background-image:url(logo.png);font-weight:bold')
+        );
+    }
+    
+    function testLenientTreatmentOfInvalidInput() {
+        // notice that we rely on the CSS validator later to fix this invalid
+        // stuff
+        $this->assertResult(
+            array('background' => 'logo.png);foo:('),
+            array('style' => 'background-image:url(logo.png);foo:();')
+        );
+    }
+    
+}
+