--id-prefix does not work on all ids

[pesca]

The id of the <a> element which contains a code block line is not prefixed.

Version:

$ pandoc --version
pandoc 2.1.2

Reproduce:

echo '```haskell
id :: a -> a
```' | pandoc --id-prefix='p'

Output:

<div class="sourceCode" id="pcb1"><pre class="sourceCode haskell"><code class="sourceCode haskell"><a class="sourceLine" id="cb1-1" data-line-number="1">id<span class="ot"> ::</span> a <span class="ot">-&gt;</span> a</a></code></pre></div>

Expected:

<div class="sourceCode" id="pcb1"><pre class="sourceCode haskell"><code class="sourceCode haskell"><a class="sourceLine" id="pcb1-1" data-line-number="1">id<span class="ot"> ::</span> a <span class="ot">-&gt;</span> a</a></code></pre></div>