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
this method was poor reading before (too deeply nested with long blocks that make determining the condition for an `else` case difficult, etc.) and is worse now.  suggest instead duplicating the `_currentBlock.Children.Add(child);` line (think that's the important one) just before the previous `continue`, with a comment about passing invalid elements through.
that is, please revert most changes to this file.