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:

saml/service_provider.go

Line 934 in bbccb79

assertionEls, err := findChildren(responseEl, "urn:oasis:names:tc:SAML:2.0:assertion", "Assertion")

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: Clever/saml2#81