Fix bug 819912 - Duplicate heading IDs

[mediocrity]

https://bugzilla.mozilla.org/show_bug.cgi?id=819912
I stole the test @darkwing created for this bug. 👯

[lmorchard]

Taking a peek... got side tracked, but tests pass. About to take a run through spot checking manually

[lmorchard]

Okay, I think I found a problem not covered in the test. The ID injection clobbers manually assigned IDs. So, this:

<h2>Section</h2>
<p>This is a section</p>
<h2 id="manual_id">Section</h2>
<p>This is a section</p>
<h2>Section</h2>
<p>This is a section</p>

Becomes this:

<h2 id="Section">Section</h2>
<p>This is a section</p>
<h2 id="Section_2">Section</h2>
<p>This is a section</p>
<h2 id="Section_3">Section</h2>
<p>This is a section</p>

But, I suspect the code was already doing this before this PR. Going to poke a little more on master branch and probably just merge this & file an issue for follow up.

[lmorchard]

Oh, I see what I did, because I wrote this code but apparently can't remember what I had for breakfast. IDs get clobbered, but name attribute can be used for a manual ID. Weird workaround, but not a new issue.