- aspnet/Razor#643 part 2: react to aspnet/Razor#653
- change test calls and delegates to include new parameter
- add tests specifically of the new feature
- add tag helpers using new feature to `TagHelpersTest` functional test
- note `HtmlEncoder`s used elsewhere e.g. in other `RazorPage` instances are unaffected
- i.e. changing one `RazorPage.HtmlEncoder` property only affects C# expressions in that page
Also simplify scope management, removing bizarre assertion when user does something odd.
nits:
- correct `// Act` and `// Act & Assert` content
- remove #YOLO wrapping
