Fix Dataloader.Ecto.run_batches/1 function for high number of batches

As stated by the Elixir doc, Maps’ keys do not follow any ordering. In practice is is especially true starting at 32 elements.
Therefore the batches' results were not in the same order as the batches' keys which resulted in an error when fetching a batch result.

lib/dataloader/ecto.ex

@@ -600,8 +600,10 @@ if Code.ensure_loaded?(Ecto) do
           on_timeout: :kill_task
         ]
 
+        batches = Enum.to_list(source.batches)
+
         results =
-          source.batches
+          batches
           |> Task.async_stream(
             fn batch ->
               id = :erlang.unique_integer()
@@ -621,7 +623,7 @@ if Code.ensure_loaded?(Ecto) do
             {:exit, reason} -> {:error, reason}
           end)
 
-        source.batches
+        batches
         |> Enum.map(fn {key, _set} -> key end)
         |> Enum.zip(results)
         |> Map.new()

test/dataloader/ecto_test.exs

@@ -456,4 +456,34 @@ defmodule Dataloader.EctoTest do
       Dataloader.get(loader, Timeout, User, user.id)
     end
   end
+
+  test "when there are many batches (more than 32)" do
+    source =
+      Dataloader.Ecto.new(
+        Repo,
+        query: fn queryable, _ ->
+          queryable
+        end
+      )
+
+    loader =
+      Dataloader.new()
+      |> Dataloader.add_source(Test, source)
+
+    users =
+      for x <- 0..50 do
+        %User{username: "Ben Wilson #{x}"} |> Repo.insert!()
+      end
+
+    loader =
+      users
+      |> Enum.reduce(loader, fn user, loader ->
+        # Force a batch per user by adding id to batch key
+        Dataloader.load(loader, Test, {User, id: user.id}, user.id)
+      end)
+      |> Dataloader.run()
+
+    assert Enum.map(users, &Dataloader.get(loader, Test, {User, id: &1.id}, &1.id)) ==
+             users
+  end
 end