Support for RubySaml 0.8.x added

Fix to support RubySaml 0.8.x.
Fixes issue: PracticallyGreen/omniauth-saml#25

The multiple value support as mentioned by @borgand should still be implemented.
Would it be possible to create a separate issue for that? As the lack of RubySaml 0.8 support blocks the upgrade of Nokogiri to name just one.