On HtmlHelper<T> we now support contextualizing an instance of
HtmlHelper<TBase> with a ViewDataDictionary<TDerived> in
ViewContext.ViewData.
This can happen in some situations when the model for a RazorPage has
been changed using a page application model convention.
In these cases we just build a new ViewDataDictionary<TBase> and pass in
the TDerived model as an instance.
