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;
 }