fix state corruption in SessionManager

lib/floof/session_manager.ex

@@ -91,12 +91,13 @@ defmodule Floof.SessionManager do
   end
 
   def get(key, subkey) do
-    Agent.get(__MODULE__, fn state ->
-      case Map.get(state, key) do
+    Agent.get(
+      __MODULE__,
+      &case Map.get(&1, key) do
         nil -> nil
         {_, m} -> Map.get(m, subkey)
       end
-    end)
+    )
   end
 
   def delete(key, subkey) do
@@ -138,7 +139,7 @@ defmodule Floof.SessionManager do
 
           case {sub, handler.(m)} do
             {nil, %{}} -> :pop
-            x -> x
+            x -> {nil, x}
           end
         end)