[asciidoc writer] html code style writes bad asciidoc block

Using pandoc 1.13.2:

HTML

```
<html lang="en">
<body>
<PRE class="badcode">No</PRE>
</body>
</html>
```

yields asciidoc:

```
code,badcode-- code,badcode
No
--
```

Note I am not sure what actually should happen with the 'badcode' class here, but this does not come out right.
