Don't use fetch access in plug.

This makes it work with structs.

lib/pleroma/plugs/authentication_plug.ex

@@ -22,7 +22,7 @@ defp verify(nil, _password) do
   end
 
   defp verify(user, password) do
-    if Comeonin.Pbkdf2.checkpw(password, user[:password_hash]) do
+    if Comeonin.Pbkdf2.checkpw(password, user.password_hash) do
       {:ok, user}
     else
       :error