Lowercase Uri for Facebook during ExecuteRetrieveAccessToken

Steps to reproduce using SimpleAuthentication.Sample.MvcAuto :
1. In SimpleAuthentication.Mvc => SimpleAuthenticationRouteConfig.cs change DefaultCallbackRoute to "Authentication/AuthenticateCallback"
2. Run the sample and log on using Facebook. This results in the error: Error validating verification code. Please make sure your redirect_uri is identical to the one you used in the OAuth dialog request

This error happens because of the different (lower vs uppercase) redirect_uri in the dialog/oauth call and the oauth/access_token call.