Using htmlparser2 results in mutilated doctype

Using cheerio 1.0.0.

```js
const cheerio = require('cheerio')
const htmlparser2 = require('htmlparser2')

const HTML = '<!DOCTYPE html><html></html>'

console.log(
  cheerio
    .load(htmlparser2.parseDocument(HTML, {}))
    .html())
```

Expected output:

```html
<!DOCTYPE html><html></html>
```

Actual output:

```html
<!DOCTYPE ><html></html>
```