Collective.saml2 and Plone 5.1

OK It was ADFS not Office 365. Not sure if that makes it compatible with ADFS.

There is a long thread about it in Single-Sign-On & Current Plone affair? - #60 by djay.

At the end was a PR to collective.saml2 which I never merged. I've now merged it but don't really have a way to test it at the moment.... so if it breaks things let me know.