Discriminate between providers when sharing an auth cookie

[Tratcher]

#859 @HaoK

This prevents [Authorize("Github")] from returning an identity produced by Facebook, etc. when they share a cookie middleware.

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[HaoK]

Can you also add a test exercising the template flow which uses a shared External cookie as opposed to named google/facebook cookies.

[Tratcher]

I don't follow, they already do use a shared cookie. https://github.com/aspnet/Security/pull/980/files#diff-f6bd2d01f6e995e0fa83a42bb849b920R1010

[HaoK]

The configure code path the templates/identity use is slightly different, they use SharedAuthenticationOptions.SignInScheme, would be good to make sure that identity/template configuration works as well with this change

[Tratcher]

SharedAuthenticationOptions is being used here: https://github.com/aspnet/Security/pull/980/files#diff-f6bd2d01f6e995e0fa83a42bb849b920R1103

[HaoK]

Ah missed that, ok looks good