Previously the extractor would return `ExtractorError::Unauthorized` when the issuer
does not provide a end_session_endpoint.
Now it will return a `ExtractorError::RpInitiatedLogoutNotSupported`.
fixed#10 by implementing a flag in the response extensions that
instructs the middleware to clear the session. The flag is automatically
set when using the `OidcRpInitiatedLogout` as a responder.
improved documentation
modified example to reflect api changes