fix: treat {:ssl_closed, _} messages as info, not warnings

They happen a lot (see benoitc/hackney#464) but
aren't worthy of a warning at the moment.

lib/concentrate/producer/http/state_machine.ex

@@ -390,7 +390,7 @@ defmodule Concentrate.Producer.HTTP.StateMachine do
 
   defp error_log_level(:closed), do: :warn
   defp error_log_level({:closed, _}), do: :warn
-  defp error_log_level({:ssl_closed, _}), do: :warn
+  defp error_log_level({:ssl_closed, _}), do: :info
   defp error_log_level(:timeout), do: :warn
   defp error_log_level({:unexpected_code, _}), do: :warn
   defp error_log_level(_), do: :error