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 diff for this file looks daunting but I really just changed:

```C#
if (tagName == null)
{
    continue;
}
```
to:
```C#
if (tagName != null)
{
...All logic that used to be skipped via continue...
}
```