No Action Defined for "#{base}/saml/attributes"

The configuration in the `README.md` has this line:
```
config.attribute_service_location = "#{base}/saml/attributes"
```
.. but neither `SamlIdp::IdpController` nor `SamlIdp::Controller` seem to define an method that is meant to be mapped to this route. 

Can the documentation be provided to offer example logic to handle this action?