Allow invalid HTML to be in Razor pages.

- Modified the TagHelperParseTreeRewriter to not remove invalid HTML snippets.
- Added tests to validate invalid HTML is allowed.

#212
revert this change.  looks like you're reversing a perf-related change @rynowak made.  existing code is definitely :100:% correct since `HtmlSymbolType` isn't a `[Flags]` `enum`.