Verify prepare callbacks in both merge directions

Author: lukaszsamson

test/ecto/changeset_test.exs

@@ -667,12 +667,11 @@ defmodule Ecto.ChangesetTest do
     cs1 = change(data) |> prepare_changes(&put_change(&1, :title, "Title"))
     cs2 = change(data) |> optimistic_lock(:upvotes)
 
-    changeset = merge(cs1, cs2)
-    assert length(changeset.prepare) == 2
-    assert changeset.filters == %{upvotes: 0}
-    assert prepared_changes(changeset) == %{title: "Title", upvotes: 1}
-
-    assert length(merge(cs2, cs1).prepare) == 2
+    for changeset <- [merge(cs1, cs2), merge(cs2, cs1)] do
+      assert length(changeset.prepare) == 2
+      assert changeset.filters == %{upvotes: 0}
+      assert prepared_changes(changeset) == %{title: "Title", upvotes: 1}
+    end
   end
 
   test "merge/2: merges types" do