updates for ADFS SAML to work

[braindev]

This is a few things I found I needed to do to to get this working with our ADFS SAML. I thought it might be worth passing it back along.

• using time.RFC3339 instead of time.RFC3339Nano (according to spec xs:dateTime https://www.oasis-open.org/committees/download.php/35711/sstc-saml-core-errata-2.0-wd-06-diff.pdf section 1.3.3 http://books.xmlschemata.org/relaxng/ch19-77049.html) for IssueInstant
• allow for unsigned requestion creation
• omitempty for AssertionConsumerServiceIndex and AttributeConsumingServiceIndex