fence renderer: fix concat of class array
fence renderer: fix concat of class array

Before:

  <pre c="l"><code>...

Now:

  <pre class="someClass language-python><code>...

Bug introduced in b7c868b64b710c8e706191d89a6dd80fcbbc9166

markdown-it/markdown-it,GerHobbelt/markdown-it,GerHobbelt/markdown-it,rlidwka/markdown-it