chore: increase test coverage

[deer]

towards #50

@hashrock, please take a look.

Additionally there's something strange going on with the coverage. It's reporting that

  image(src: string, title: string | null, alt: string | null) {
    return `<img src="${src}" alt="${alt ?? ""}" title="${title ?? ""}" />`;
  }

isn't covered, even though this is definitely executed. There are a few other spots that have the same problem.

[hashrock]

awesome thank you deer!
I find it strange too, but I'll keep it in mind

[deer]

@hashrock, do you know if there's an issue for this coverage problem in https://github.com/denoland/deno? To clarify: if I set a breakpoint within

  image(src: string, title: string | null, alt: string | null) {
    return `<img src="${src}" alt="${alt ?? ""}" title="${title ?? ""}" />`;
  }

it catches during the test, but the coverage report shows it as uncovered.

If you don't know of one, then I'll log an issue. (I've checked myself, and I don't see one, but maybe I'm missing something.)

[hashrock]

@deer sorry. I don't usually use the coverage command, so I don't know if there are similar issues. I did a quick search but couldn't find any similar issues, so please feel free to file new one.