Make the relay_state optional in the response.

[matejak]

If relay_state isn't part of the outgoing request, it won't come back as a response. In that case, the code wouldn't work.

I have renamed the relay_state to its semantic meaning redirect_to, and adjusted the code so that it conforms with the method's comments.

This behavior occurs when SERVER_NAME is not specified in the configuration, which also affects get_login_return_url though - in that case, no URL will pass the is_valid_redirect_url validation, as the code calls make_absolute_url which needs the server name set to work properly.