title: devise_saml_authenticatable response does not contain attributes
body: <p>I'm in the process of trying to understand SSO, SAML, and federated identity management concepts. Right now I'm trying to authenticate with my app using <a href="https://github.com/apokalipto/devise_saml_authenticatable" rel="nofollow">devise_saml_authenticatable</a> as SP; and SSOCircle as an IDP. I am getting a saml_response with nil attributes, looking at the gem code, I see that it might have to do with saml_config having no attributes in the consumer service. My guess is that it has to do with not having a <a href="https://github.com/apokalipto/devise_saml_authenticatable/blob/b4e6ed56a766a7489cebe4852033900419454784/lib/devise_saml_authenticatable/saml_config.rb" rel="nofollow"><code>config/idp.yml</code></a> file. Does anyone know how this <code>idp.yml</code> file should look like, what it should contain? I cannot find it in the repo documentation.</p>