`saml:Assertion` is not found without a namespace

[LauraBeatris]

Some IdPs return saml:Assertion without the xmlns:saml="urn:oasis:names:tc:SAML:2.0:assertion" namespace, and therefore it fails to query for it here:

https://github.com/crewjam/saml/blob/bbccb7933d5f60512ebc6caec7120c604581983d/service_provider.go#L934

We should handle the case where namespace isn't directly applied to Assertion tag and read all the namespaces from the Response element onto the Assertion element.

Refer to this implementation: https://github.com/Clever/saml2/pull/81