fix(pairing): ensure TO2 messages have correct content type (#1907)

phoenix sometimes adds charset=utf8 to the content-type, which is not
compatible with our clients. manually set it to "application/cbor"
to avoid issues altogether

Signed-off-by: Francesco Noacco <francesco.noacco@secomind.com>

Author: noaccOS

apps/astarte_pairing/lib/astarte_pairing_web/plug/setup_fdo.ex

@@ -43,6 +43,7 @@ defmodule Astarte.PairingWeb.Plug.SetupFDO do
     case read_body(conn) do
       {:ok, body, conn} ->
         conn
+        |> put_resp_header("content-type", "application/cbor")
         |> put_resp_header("message-type", to_string(next_message_id))
         |> assign(:message_id, message_id)
         |> assign(:cbor_body, body)