Add settings.protocol_binding attribute with tests. Allows setting of Pr...

...otocolBinding attribute in AuthnRequest root node of xml doc.

I made this change a while ago. I noticed that in your commit 7e910324efa8e5d15eef8ba6a472d136d0ab4df5, you made almost the exact same change, but mine also includes tests. Would you mind merging my changes to get the tests and so I can get back on your official gem?