implement fix for #10

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

src/error.rs

@@ -13,6 +13,9 @@ pub enum ExtractorError {
 
     #[error("rp initiated logout information not found")]
     RpInitiatedLogoutInformationNotFound,
+
+    #[error("could not build rp initiated logout uri")]
+    FailedToCreateRpInitiatedLogoutUri,
 }
 
 #[derive(Debug, Error)]
@@ -88,6 +91,9 @@ impl IntoResponse for ExtractorError {
             Self::RpInitiatedLogoutInformationNotFound => {
                 (StatusCode::INTERNAL_SERVER_ERROR, "intenal server error").into_response()
             }
+            Self::FailedToCreateRpInitiatedLogoutUri => {
+                (StatusCode::INTERNAL_SERVER_ERROR, "intenal server error").into_response()
+            }
         }
     }
 }