Fixed mixin calls not working from dynamic mixins

Dynamic mixins aren't treated enough like Rulesets.
There is some code duplication which needs to be cleaned up,
ideally they should share a prototype.

test/css/mixins-nested.css

@@ -3,4 +3,5 @@
 }
 .class .inner .innest {
   width: 30;
+  border-width: 60;
 }

test/less/mixins-nested.less

@@ -1,9 +1,14 @@
+.mix-inner (@var) {
+  border-width: @var;
+}
+
 .mix (@a: 10) {
   .inner {
     height: @a * 10;
 
     .innest {
       width: @a;  
+      .mix-inner(@a * 2);
     }  
   }
 }