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.

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)
+```
+```