fixes svg-gradient function broken by prev. color object changes

Author: seven-phases-max

lib/less/functions/svg.js

@@ -1,6 +1,7 @@
 module.exports = function(environment) {
     var Dimension = require("../tree/dimension"),
         Color = require("../tree/color"),
+        Expression = require("../tree/expression"),
         Quoted = require("../tree/quoted"),
         URL = require("../tree/url"),
         functionRegistry = require("./function-registry");
@@ -50,7 +51,7 @@ module.exports = function(environment) {
             '<' + gradientType + 'Gradient id="gradient" gradientUnits="userSpaceOnUse" ' + gradientDirectionSvg + '>';
 
         for (i = 0; i < stops.length; i+= 1) {
-            if (stops[i].value) {
+            if (stops[i] instanceof Expression) {
                 color = stops[i].value[0];
                 position = stops[i].value[1];
             } else {