Don't lower-case components of background.

Signed-off-by: Edward Z. Yang <ezyang@mit.edu>

library/HTMLPurifier/AttrDef/CSS/Background.php

@@ -32,7 +32,7 @@ class HTMLPurifier_AttrDef_CSS_Background extends HTMLPurifier_AttrDef
         $string = $this->mungeRgb($string);
 
         // assumes URI doesn't have spaces in it
-        $bits = explode(' ', strtolower($string)); // bits to process
+        $bits = explode(' ', $string); // bits to process
 
         $caught = array();
         $caught['color']    = false;