Better "logout" support by sending a 401 response after logout

[agwells]

As mentioned in the README, browsers tend to cache successful HTTP auth credentials indefinitely, which makes it difficult to really "log out" if you're a human being using this plugin.

However, testing in FF and Chrome indicates that they will clear cached credentials, if they receive a 401 response to those creds. So, for use cases where people might want to use this plugin to authenticate human beings into a site, it would be useful to have the option of sending a 401 response after the user logs out.

(Another method to clear the cached credentials, is to put an invalid username in the URL with @, e.g. https://nosuchuser@moodle.example.com . This will also cause the browser to forget existing, previously-successful HTTP auth credentials for that domain. But this can cause some weird edge case behavior in the UI on page refreshes and such.)