Fixes the billion laughs XML vulnerability.

Instead of using the Hash.from_xml(xml) method of ActiveSupport, use the ActiveSupport::XmlMini_REXML.parse(xml) method. This ensures that invalid XML messages with the billion laughs vulnerability still raise an RuntimeError.

Move the ‘fix’ from the Saml.parse_message method to the Saml::Base::HappyMapperClassMethods.parse method.

Use a valid XML message for testing the vulnerability.