The solution was in the AuthToken from my code, I didn't squeezed the final '\n' character !
Later when the http request was generated, headers were split, making the GFE returning 400 Bad Request.
This is the corrected 'extract_auth/1':
extract_auth(<<"Auth=", Rest/binary>>) ->
Size = size(Rest) - 1,
<<Auth:Size/binary, _/binary>> = Rest,
I squeeze the final character !
Here's a link to the google groups discussion.