Add SessionNotOnOrAfter attribute to Saml::Elements::AuthnStatement

[truongnmt]

What

Add SessionNotOnOrAfter attribute to Saml::Elements::AuthnStatement.

Why

From SAML core spec 2.7.2:

SessionNotOnOrAfter [Optional]

Specifies a time instant at which the session between the principal identified by the subject and the SAML authority issuing this statement MUST be considered ended. The time value is encoded in UTC, as described in Section 1.3.3.

This solve #172

[coveralls]

Coverage decreased (-0.0002%) to 99.848% when pulling 86ab4d8 on truongnmt:add-session_not_on_or_after-attribute-to-authn_statement into a09fe2f on digidentity:master.

[jdongelmans]

Thanks, version v3.9.1 has just been pushed!