'Create an object that represents a SAML Assertion.
<ns0:Assertion
ID="35daed258ba647ba8962e9baff4d6a46"
IssueInstant="2014-06-11T15:45:58Z"
Version="2.0">
<ns0:Issuer> ... </ns0:Issuer>
<ns1:Signature> ... </ns1:Signature>
<ns0:Subject> ... </ns0:Subject>
<ns0:AuthnStatement> ... </ns0:AuthnStatement>
<ns0:AttributeStatement> ... </ns0:AttributeStatement>
</ns0:Assertion>
:returns: XML <Assertion> object'

def _create_assertion(self, issuer, signature, subject, authn_statement, attribute_statement):

    assertion = saml.Assertion()
    assertion.id = self.assertion_id
    assertion.issue_instant = utils.isotime()
    assertion.version = '2.0'
    assertion.issuer = issuer
    assertion.signature = signature
    assertion.subject = subject
    assertion.authn_statement = authn_statement
    assertion.attribute_statement = attribute_statement
    return assertion
