pipeline_test: Fix usage of %Activity{}

lib/pleroma/web/activity_pub/object_validator.ex

@@ -169,7 +169,7 @@ defmodule Pleroma.Web.ActivityPub.ObjectValidator do
 
   def cast_and_apply(o), do: {:error, {:validator_not_set, o}}
 
-  # is_struct/1 isn't present in Elixir 1.8.x
+  # is_struct/1 appears in Elixir 1.11
   def stringify_keys(%{__struct__: _} = object) do
     object
     |> Map.from_struct()

lib/pleroma/web/activity_pub/pipeline.ex

@@ -40,6 +40,8 @@ defmodule Pleroma.Web.ActivityPub.Pipeline do
     end
   end
 
+  def do_common_pipeline(%{__struct__: _}, _meta), do: {:error, :is_struct}
+
   def do_common_pipeline(object, meta) do
     with {_, {:ok, validated_object, meta}} <-
            {:validate_object, @object_validator.validate(object, meta)},