Add logout param support

[Jaywoods2]

after logout,i can redirect to the login page and login.

[geoffgarside]

From the current CAS spec it looks like it should be an option of the CAS client application whether or not to send a service parameter with the /logout request. It would be better if this was therefore an option rather than always sending a service URL.

[Jaywoods2]

After listening to your opinions,I have modified the code that adds a judgment parameters，and it can choose whether to send the service parameters。Do you think so? I hope you can give me more advice！

[geoffgarside]

I was thinking of leaving the LogoutUrlForRequest as it is at the moment and adding LogoutUrlForRequestWithService(r *http.Request, serviceURL *url.URL) (string, error) allowing the serviceURL to be nil to enable the URL to be extracted from the http.Request.

[geoffgarside]

Actually I think having the client-wide option is good. What would happen though if someone was to hit the applications "/logout" route, would this not result in the user being sent to something like

https://sso.example.com/logout?service=https://app.example.com/logout

which would just result in another logout, so would the logout URL code need to sanitise the URL so the service URL would be sent as https://app.example.com/ instead?

[geoffgarside]

Taken me far too long to come back to this...