Enum.count on has_many associations shouldn't break

Hey, while upgrading a pet project of mine to the latest elixir/dynamo/ecto bits I stumbled over a regression in our app. 

Apparently the Enumerable protocol has changed for the count function. It is now required to return a tuple {:ok, value} instead of value alone. 

I added also a test for this regression.

Cheerio
Björn