Fix oauth new behavior after upgrading to v2

lib/stuart_client_elixir/authenticator.ex

@@ -48,6 +48,7 @@ defmodule StuartClientElixir.Authenticator do
       client_secret: client_secret,
       site: site
     )
+    |> OAuth2.Client.put_serializer("application/json", Jason)
   end
 
   defp cache_exists?(%Credentials{client_id: client_id}),

test/stuart_client_elixir/authenticator_test.exs

@@ -19,6 +19,9 @@ defmodule StuartClientElixirTest.AuthenticatorTest do
         new: fn oauth_params ->
           sample_client(oauth_params)
         end,
+        put_serializer: fn oauth_client, content_type, serializer ->
+          client_with_serializer(oauth_client, content_type, serializer)
+        end,
         get_token: fn oauth_client ->
           get_token_response(oauth_client)
         end
@@ -158,6 +161,10 @@ defmodule StuartClientElixirTest.AuthenticatorTest do
     }
   end
 
+  defp client_with_serializer(oauth_client, content_type, serializer) do
+    %{oauth_client | serializers: %{content_type => serializer}}
+  end
+
   defp get_token_response(%OAuth2.Client{client_secret: "client-secret"}) do
     {:ok,
      sample_client_with_token(