Add SessionIndex to sp-initiated logout request

[franklinjjeng]

Adding SessionIndex to sp-initiated logout request. Updating defaultLogoutRequestTemplate context to include: <samlp:SessionIndex>{SessionIndex}</samlp:SessionIndex>

Correcting flow.ts -> sp.createLogoutRequest example. user object should be using logoutNameID instead of email.

[coveralls]

Coverage remained the same at 89.75% when pulling d2d55f5 on franklinjjeng:master into 031cafd on tngan:master.

[coveralls]

Coverage remained the same at 89.75% when pulling d2d55f5 on franklinjjeng:master into 031cafd on tngan:master.

[franklinjjeng]

Let me know if I was supposed to put a PR to a different branch or if you'd like to make SessionIndex appear conditionally if it's not always expected as it may cause issues.

[tngan]

@franklinjjeng In accordance of section 3.7 of specification, session index is optional.

Our default template supposes to include required fields. You can use custom template to handle this use case.