Fix as_html spec and add spec for _as_html

[joshsmith]

Made these changes attempting to fix dialyzer warnings I'm getting for as_html and as_html!.

This doesn't fix those, but should be helpful.

[RobertDober]

Thank you. I have to admit that getting mix dializer to work was on my TODO list because it would be of great value for potential contributers, given my current lack of time, that seems important.

But given my current lack of time it does not progress.

[joshsmith]

C’est cool! Je comprends certainement. 🙂