Occasional `KeyError` during the federated login process

Sometimes when I login to a federated account I get a KeyError on either next or issuer after logging in

Thrown by these lines:

• https://git.startinblox.com/djangoldp-packages/djangoldp-account/blob/master/djangoldp_account/endpoints/rp_login.py#L205
• https://git.startinblox.com/djangoldp-packages/djangoldp-account/blob/master/djangoldp_account/endpoints/rp_login.py#L88

This may be caused by using a private browsing session

The first is easy to handle - use getattr and default to settings.SITE_URL

The second is not so easy to handle...