Fix dynamic mixins with nested rules

Dynamic mixins with more than one level of nesting wouldn't work.
This is now fixed. Also refactored mixin.definition.eval a little.

lib/less/tree/ruleset.js

@@ -7,6 +7,12 @@ tree.Ruleset = function Ruleset(selectors, rules) {
 };
 tree.Ruleset.prototype = {
     eval: function () { return this },
+    evalRules: function (context) {
+        return new(tree.Ruleset)(this.selectors, this.rules.map(function (r) {
+            if (r.evalRules) return r.evalRules(context);
+            else             return r.eval(context);
+        }));
+    },
     variables: function () {
         if (this._variables) { return this._variables }
         else {