- `IRazorViewEngine.MakePathAbsolute()` -> `GetAbsolutePath()`
- set `IsPartial` on all `IRazorPage` instances
- improve consistency of methods in `HtmlHelperPartialExtensions`
- a couple unnecessarily passed `htmlHelper.ViewData`
- add missing tests of these extension methods
- restore parameter checks in `CompositeViewEngine`
- reduce `List<string>` and remove enumerator allocations in `CompositeViewEngine`
nits:
- correct a few comments
- use `<seealso/>`
