Properly invalidate all_embedding_documents on creation of an embedded document

cc: @mzikherman

Before, we would've had this undesirable behavior:

```ruby
class Superfoo
  include Mongoid::Document  
  embeds_many :foos  
end  

class Foo
  include Mongoid::Document  
  embedded_in :superfoo  
end  

def find_cached_superfoo(id)
  Garner::Cache::ObjectIdentity.cache({ bind: [Superfoo, { id: id }] }) do  
    Superfoo.find(id)    
  end    
end  

superfoo = Superfoo.create!
puts find_cached_superfoo(superfoo.id).foos
# >> []

superfoo.foos.create!
puts superfoo.foos
# >> [#<Foo _id: 519c237a4baef8f07c000004, >]
puts find_cached_superfoo(superfoo.id).foos
# >> []
```