use correct arity .post.

lib/pleroma/web/salmon/salmon.ex

@@ -137,7 +137,7 @@ defmodule Pleroma.Web.Salmon do
 
   defp send_to_user(_,_,_), do: nil
 
-  def publish(user, activity, poster \\ &@httpoison.post/3)
+  def publish(user, activity, poster \\ &@httpoison.post/4)
   def publish(%{info: %{"keys" => keys}} = user, activity, poster) do
     feed = ActivityRepresenter.to_simple_form(activity, user, true)
     |> ActivityRepresenter.wrap_with_entry