Fail to detect coding block when there is a space gap between `<pre> <code>`.

[Norlandz]

The current implementation uses node.firstChild,
maybe using node.firstElementChild is better for cases where there is a space gap between <pre> <code>?

rules.fencedCodeBlock = {
  filter: function (node, options) {
    return (
      options.codeBlockStyle === 'fenced' &&
      node.nodeName === 'PRE' &&
      node.firstChild &&
      node.firstChild.nodeName === 'CODE'
    )
  },

https://github.com/mixmark-io/turndown/blob/master/src/commonmark-rules.js