Stringify Ecto.Enum values when using string_params/string_params_with_assocs

[aleDsz]

I've been using ExMachina for so long, and then my recent project is using the Ecto.Enum feature, which when I tried to use string_params_with_assocs/2 (the same happens with string_params_for/2), it didn't change the field type to string, it kept as atom.

def post_factory do
  %MyApp.Post{
    type: :news
  }
end

iex> params_for(:post)
# => %{post: :news}

iex> string_params_for(:post)
# => %{"post" => :news}

I would be happy to send a PR to make it (optionally) stringfiable.