fix utf8 characters serialization issue

lib/exvcr/handler.ex

@@ -140,6 +140,7 @@ defmodule ExVCR.Handler do
   defp get_response_from_server(request, recorder, record?) do
     adapter = ExVCR.Recorder.options(recorder)[:adapter]
     response = :meck.passthrough(request)
+               |> convert_body
                |> adapter.hook_response_from_server
     if record? do
       raise_error_if_cassette_already_exists(recorder, inspect(request))
@@ -149,6 +150,11 @@ defmodule ExVCR.Handler do
     response
   end
 
+  defp convert_body(response) do
+    {status, code, headers, body} = response
+    {status, code, headers, IO.iodata_to_binary(body)}
+  end
+
   defp ignore_request?(request, recorder) do
     ignore_localhost = ExVCR.Recorder.options(recorder)[:ignore_localhost] || ExVCR.Setting.get(:ignore_localhost)
     if ignore_localhost do