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
@davidfowl FYI @rynowak found Razor had enough tight loops that the Reflection `HasFlag()` does had a visible performance impact.  This particular doesn't run on every character (just every open tag) but the thought was to be consistent w/in Razor.