Add settings.protocol_binding attribute with tests. 

Allows setting of ProtocolBinding attribute in AuthnRequest root node of xml doc.

I made this change a while ago. I noticed that you guys made commit 7e910324efa8e5d15eef8ba6a472d136d0ab4df5 the very next day with 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?