removed attributes from child headings inside <div>

[ousia]

I accidentally discovered that titles inside <div> get their attributes removed.

Sample:

<div>
# [Title with attributes] {.hidden #hammer}
## [another title] {.attr #noma}
</div>

<div class="matter">
# [Title with attributes] {.hidden #hammer}
### [another title] {.attr #noma}
</div>

And HTML ouput from latest stable pandoc:

<div>
<h1>[Title with attributes]</h1>
<h2>[another title]</h2>
</div>
<div class="matter">
<h1>[Title with attributes]</h1>
<h3>[another title]</h3>
</div>

Could you fix this?

[jgm]

Dup of #1711, fixed already by e16683b.