HTML writer: Fixed footnote backlinks with --id-prefix.

Closes #4235.

src/Text/Pandoc/Writers/HTML.hs

@@ -1150,7 +1150,7 @@ blockListToNote :: PandocMonad m => WriterOptions -> String -> [Block] -> StateT
 blockListToNote opts ref blocks =
   -- If last block is Para or Plain, include the backlink at the end of
   -- that block. Otherwise, insert a new Plain block with the backlink.
-  let backlink = [Link ("",["footnote-back"],[]) [Str "↩"] ("#" ++ writerIdentifierPrefix opts ++ "fnref" ++ ref,[])]
+  let backlink = [Link ("",["footnote-back"],[]) [Str "↩"] ("#" ++ "fnref" ++ ref,[])]
       blocks'  = if null blocks
                     then []
                     else let lastBlock   = last blocks

test/command/4235.md

@@ -0,0 +1,12 @@
+```
+% pandoc --id-prefix='foo'
+This.^[Has a footnote.]
+^D
+<p>This.<a href="#foofn1" class="footnote-ref" id="foofnref1"><sup>1</sup></a></p>
+<section class="footnotes">
+<hr />
+<ol>
+<li id="foofn1"><p>Has a footnote.<a href="#foofnref1" class="footnote-back">↩</a></p></li>
+</ol>
+</section>
+```