user-configured redirect URL clobbered in oauthproxy.go

[dt-rush]

see line 55 of oauthproxy.go (link)

We literally overwrite whatever the value is with fmt.Sprintf("%s/callback", opts.ProxyPrefix) even if the user provided a redirect URL they want to use.

[dt-rush]

This PR will be sent to pusher's fork since this repo is abandoned by bitly ( see #628 )

[ploxiln]

I think the reason for the current behavior, is to allow the option to set the scheme://domain, but the path should be known by oauth2_proxy pretty well, so it overwrites just the path.

[ploxiln]

(agreed it's a odd to ignore the path in the option, maybe it's trying to silently fix mistakes in a manually set callback url path)

[dt-rush]

As far as I see it, the user specifying a callback URL should specify the callback URL, not the callback scheme://domain.

[dt-rush]

Actually there are more problems here in any case. OAuthCallbackPath is used to handle the callback, not RedirectURL... these are supposed to mean the same thing.