Reliably convert colors to hex strings

Converting a hex string to CPColor and back to a hex string now returns
the same initial hex string. (Fixed an off-by-one error)

AppKit/CPColor.j

@@ -736,7 +736,7 @@ var byteToHex = function(n)
     if (!n || isNaN(n))
         return "00";
 
-    n = ROUND(MIN(255, MAX(0, 256 * n)));
+    n = FLOOR(MIN(255, MAX(0, 256 * n)));
 
     return hexCharacters.charAt((n - n % 16) / 16) +
            hexCharacters.charAt(n % 16);