Support http 302 redirect responses in get_dir_contents

I'm seeing http 302 responses for the Repo.get_dir_contents function call:
http://jacquev6.github.io/PyGithub/github_objects/Repository.html#github.Repository.Repository.get_dir_contents

I don't know if any of this is releated, but it is a private repo owned by an organization (not a user).  

I had originalized gone to add the redirect handling at a lower level, but after some experimentation this seems to be the only function call that's exhibiting this behavior.