Add bypass for saturate(5%) filter function. Helps issue #1299

lib/less/functions.js

@@ -96,6 +96,11 @@ tree.functions = {
         return new(tree.Dimension)(Math.round(color.luma() * color.alpha * 100), '%');
     },
     saturate: function (color, amount) {
+        // filter: saturate(3.2);
+        // should be kept as is, so check for color
+        if (!color.rgb) {
+            return null;
+        }
         var hsl = color.toHSL();
 
         hsl.s += amount.value / 100;