SignInAsync sets Location header

When calling SignInAsync on the login page, the Location header gets set to the value of the returnUrl query string parameter.

https://github.com/aspnet/AspNetCore/blob/master/src/Security/Authentication/Cookies/src/CookieAuthenticationHandler.cs#L334-L335

Besides I am unclear why this would be ever useful, this is a non-obvious side effect of calling SignIn and interferes with other logic on the login page. You might not always want to do a redirect - but maybe show a view. Calling `return View(...);` will not work because the response has already been modified to be a redirect.