Cookie MW null reference exception

[dolly22]

I'm getting null reference exception from cookie authentication middleware when protecting authentication ticket. This happens when the following conditions occur on request:

1. Sliding expiration is enabled and enough time passed so middleware tries to refresh authentication ticket (setting _shouldRenew = true)
2. Principal is rejected in Options.Notifications.ValidatePrincipal notification (setting Context.Principal = null)
3. _shouldRenew path is hit inside ApplyResponseGrantAsync() and null reference is thrown from Options.TicketDataFormat.Protect(model); because Principal was set to null.

Also Security/src/Microsoft.AspNet.Authentication.Cookies/Notifications/CookieValidateIdentityContext.cs should be probably renamed to CookieValidatePrincipalContext.cs

[muratg]

@HaoK Could you have a look? We can move it out of Beta 6 based on your workload.

[HaoK]

d7ce42d