Very simple preformatted code block does not get converted properly about turndown

[domchristie]

This looks like it's working on the live demo: https://mixmark-io.github.io/turndown/
The output is:

```

fruits = ["apple", "banana", "cherry"]

for fruit in fruits:
  print(fruit)
```

[dddoyle1]

Looks like this happens only when I pass the HTML element directly, whereas if I instead do

turndownService.turndown(element.outerHTML)

the element is converted as I expected. Is this intended behavior?

FWIW, I see that when I pass the element directly, the thing that makes it to fencedCodeBlock.filter is a node with "<pre>...</pre>" stripped away, ie a code node, which fails the filter criteria

[XinBaoCode]

The codeBlockStyle option will do this for you. For example:

var turndownService = new TurndownService({ codeBlockStyle: 'fenced' })
turndownService.turndown('<pre><code class="language-js">console.log("hello world")</code></pre>')