- #643 part 1
- change is viral and requires an update to `RazorPage.StartTagHelperWritingScope()`
- memoize `GetChildContentAsync()` per-encoder
- update generation tests to match and to test new behaviour
- note `HtmlEncoder`s used elsewhere e.g. in other `RazorPage` instances are unaffected
Add `NullHtmlEncoder`
Nits:
- generally clean up affected doc comments and make them more consistent
- remove unused `using`s in files I had open
Doug Bunting
N. Taylor Mullen