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
the only change on this line is `sym.Type == type` to `type.HasFlag(sym.Type)`.  that reverses a performance improvement @rynowak made, isn't relevant in this PR, and impacts only the performance (not the semantics).