Callbacks are fired multipled times for nested embedded_many relationships

I tried to fix this but hit a wall. The closest I got was: 

```ruby
--- a/lib/mongoid/callbacks.rb
+++ b/lib/mongoid/callbacks.rb
@@ -169,7 +169,6 @@ module Mongoid
           Array.wrap(relation).each do |child|
             next if children.include?(child)
             children.add(child) if cascadable_child?(kind, child)
-            child.send(:cascadable_children, kind, children)
           end
         end
       end
```

but this causes `spec/mongoid/nested_attributes_spec.rb:709` to fail