Prevent proxyCallbackUrl from being rewritten by default configuration

When specifying an explicit proxyCallbackUrl in Web.config:

https://app.example.com/Account/ProxyCallbackUrl?proxyResponse=true

The base path should have been /Account/ProxyCallbackUrl but instead was being rewritten as /Account/CurrentMethod where CurrentMethod could be any route the client is currently browsing. The corresponding error in cas.log then becomes:

InvalidProxyChainTicketValidationException: Invalid proxy chain: https://app.example.com/Account/Restricted?proxyResponse=true

The proposed file change effectively "scopes" the setting of the base path when and only when no CasAuthentication.CasProxyCallbackUrl has been specified and resolves the issue:

[org.jasig.cas.client.validation.Cas20ProxyReceivingTicketValidationFilter] - Successfully authenticated user: jdoe

See pull request here: https://github.com/Jasig/dotnet-cas-client/pull/21