Markdown writer: Fix regression in code blocks with attributes.

Code blocks with a single class but nonempty attributes
were having attributes drop as a result of #7242.

Closes #7397.

src/Text/Pandoc/Writers/Markdown.hs

@@ -256,10 +256,10 @@ noteToMarkdown opts num blocks = do
               then hang (writerTabStop opts) (marker <> spacer) contents
               else marker <> spacer <> contents
 
--- | (Code) blocks with a single class can just use it standalone,
--- no need to bother with curly braces.
+-- | (Code) blocks with a single class and no attributes can just use it
+-- standalone, no need to bother with curly braces.
 classOrAttrsToMarkdown :: Attr -> Doc Text
-classOrAttrsToMarkdown ("",[cls],_) = literal cls
+classOrAttrsToMarkdown ("",[cls],[]) = literal cls
 classOrAttrsToMarkdown attrs = attrsToMarkdown attrs
 
 -- | Ordered list start parser for use in Para below.

test/command/7397.md

@@ -0,0 +1,14 @@
+```
+% pandoc -t markdown
+~~~~ { .haskell startFrom="100"}
+qsort []     = []
+qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++
+               qsort (filter (>= x) xs)
+~~~~
+^D
+``` {.haskell startFrom="100"}
+qsort []     = []
+qsort (x:xs) = qsort (filter (< x) xs) ++ [x] ++
+               qsort (filter (>= x) xs)
+```
+```