Middleware does not fire when Request.PathBase is not empty

[MrDustpan]

Hello,
I'm running into an issue where the middleware doesn't fire on a site that is running under an application in IIS, for example: http://localhost/MySite/MyCallback.

I believe this is due to a difference in the way the CallbackPath is checked between WsFederationAuthenticationHandler and RemoteAuthenticationHandler:

https://github.com/chrisdrobison/aspnetcore-wsfed/blob/master/AspNetCore.Authentication.WsFederation/WsFederationAuthenticationHandler.cs#L37
vs.
https://github.com/aspnet/Security/blob/rel/1.1.3/src/Microsoft.AspNetCore.Authentication/RemoteAuthenticationHandler.cs#L26

As you can see, the base class does not include the Request.PathBase in its logic. Do you think it would make sense to change the WsFederationAuthenticationHandler to match this logic? If so I'd be happy to provide a PR.