fix: invalid MapSet usage

lib/floof.ex

@@ -95,7 +95,7 @@ defmodule Floof do
           {backlog, sesskey}
 
         {:fwddrop, dcdhash} ->
-          to_drop = coalesce_fwddrops(MapSet.new(dcdhash))
+          to_drop = coalesce_fwddrops(MapSet.new([dcdhash]))
           :ok = send_summary(client, :drop, to_drop)
           {backlog, sesskey}
 

lib/floof/session_manager.ex

@@ -101,6 +101,8 @@ defmodule Floof.SessionManager do
         end
       )
 
+    filter_keys = MapSet.delete(filter_keys, nil)
+
     {:ok, all_keys} = all_session_keys(state)
 
     # filter_keys contains all subscribed sessions