evaluate arguments, before passing them to a function

lib/less/node/call.js

@@ -6,6 +6,7 @@ tree.Call = function Call(name, args) {
 };
 tree.Call.prototype = {
     toCSS: function (context, env) {
-        return tree.functions[this.name].apply(tree.functions, this.args).toCSS();
+        var args = this.args.map(function (a) { return a.eval() });
+        return tree.functions[this.name].apply(tree.functions, args).toCSS();
     }
 };