Allow setting associations to nil

Previously the catchall was falling through to the list case and trying
to enumerate `nil`. This commit adds a guard to the list condition to
ensure that doesn't happen again, then adds a new condition to handle
`nil`.

I _think_ this should be handling all cases now, so it also removes the
catchall. This way, if it fails in a way we didn't expect, it should
give an obvious failure that there was no matching case.

Fixes #191

lib/ex_machina/ecto_strategy.ex

@@ -82,13 +82,19 @@ defmodule ExMachina.EctoStrategy do
     case original_assoc do
       %{__meta__: %{__struct__: Ecto.Schema.Metadata, state: :built}} ->
         cast(original_assoc)
+
       %{__struct__: _} ->
         original_assoc
+
       %{} ->
         assoc_type = schema.__schema__(:association, assoc).related
         assoc_type |> struct |> Map.merge(original_assoc) |> cast
-      _list ->
+
+      list when is_list(list)->
         Enum.map(original_assoc, &(cast(&1)))
+
+      nil ->
+        nil
     end
   end
 end

test/ex_machina/ecto_strategy_test.exs

@@ -65,6 +65,12 @@ defmodule ExMachina.EctoStrategyTest do
     assert user.id != nil
   end
 
+  test "insert/1 sets nil values" do
+    model = TestFactory.insert(:article, author: nil)
+
+    assert model.author == nil
+  end
+
   test "insert/1 casts bare maps" do
     model = TestFactory.insert(:article, author: %{net_worth: 300})