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
https://github.com/aspnet/Razor/pull/213/files#diff-c7263bc61a2f04220f59ba23437f145bR141
https://github.com/aspnet/Razor/pull/213/files#diff-c7263bc61a2f04220f59ba23437f145bR189
https://github.com/aspnet/Razor/pull/213/files#diff-c7263bc61a2f04220f59ba23437f145bR201