Raw EPUB attributes are rendered in EPUB output.

```
% pandoc -f native -t epub --metadata title=Raw -o - | pandoc -f epub -t html
[ RawBlock (Format "epub") "<p>ok</p>" ]
^D
<p><span id="title_page.xhtml"></span></p>
<p><span id="ch001.xhtml"></span></p>
<section id="ch001.xhtml_raw" class="unnumbered level1">
<h1 class="unnumbered">Raw</h1>
<p>ok</p>
</section>
```

Raw EPUB2 attributes are omitted from EPUB3 output.

```
% pandoc -f markdown -t epub3 --metadata title=Raw -o - | pandoc -f epub -t html
~~~ {=epub2}
<p>ok</p>
~~~
^D
<p><span id="title_page.xhtml"></span></p>
<p><span id="ch001.xhtml"></span></p>
<section id="ch001.xhtml_raw" class="unnumbered level1">
<h1 class="unnumbered">Raw</h1>
</section>
```

```
% pandoc -f markdown -t epub3 --metadata title=Raw -o - | pandoc -f epub -t html
~~~ {=epub3}
<p>ok</p>
~~~
^D
<p><span id="title_page.xhtml"></span></p>
<p><span id="ch001.xhtml"></span></p>
<section id="ch001.xhtml_raw" class="unnumbered level1">
<h1 class="unnumbered">Raw</h1>
<p>ok</p>
</section>
```