Fix Bandit.HTTP1.Adapter.inform/3 return value (#268)

* Fix Bandit.HTTP1.Adapter.inform/3 return value

Per https://hexdocs.pm/plug/Plug.Conn.Adapter.html#c:inform/3, the
callback should return:

    :ok | {:error, :not_supported}

The way Plug.Conn.inform! is implemented at the moment,
https://github.com/elixir-plug/plug/blob/v1.15.2/lib/plug/conn.ex#L1335,
it looked like Bandit did _not_ implement inform but it sure does!

* Update

lib/bandit/http1/adapter.ex

@@ -517,7 +517,7 @@ defmodule Bandit.HTTP1.Adapter do
       |> Map.put(:resp_start_time, start_time)
       |> Map.put(:resp_body_bytes, 0)
 
-    {:ok, nil, %{req | metrics: metrics}}
+    {:ok, %{req | metrics: metrics}}
   end
 
   defp response_header(nil, status, headers), do: response_header("HTTP/1.0", status, headers)