Respect empty LineBlock lines in plain writer

The plain writer behaved as a markdown variant with Ext_line_blocks
turned off, and so empty lines in a line block would get eliminated.
This is surprising, since if there's anything where the intent can be
preserved in plain text output it's empty lines.

It's still a bit surprising to have nbsps in plain text output, as in
the test, where the distinction doesn't really matter, but that'd be an
orthogonal change.

src/Text/Pandoc/Writers/Markdown.hs

@@ -21,7 +21,7 @@ module Text.Pandoc.Writers.Markdown (
   writeCommonMark,
   writeMarkua,
   writePlain) where
-import Control.Monad (foldM, zipWithM, MonadPlus(..), when)
+import Control.Monad (foldM, zipWithM, MonadPlus(..), when, liftM)
 import Control.Monad.Reader ( asks, MonadReader(local) )
 import Control.Monad.State.Strict ( gets, modify )
 import Data.Default
@@ -447,12 +447,19 @@ blockToMarkdown' opts (Plain inlines) = do
   return $ contents <> cr
 blockToMarkdown' opts (Para inlines) =
   (<> blankline) `fmap` blockToMarkdown opts (Plain inlines)
-blockToMarkdown' opts (LineBlock lns) =
-  if isEnabled Ext_line_blocks opts
-  then do
-    mdLines <- mapM (inlineListToMarkdown opts) lns
-    return $ vcat (map (hang 2 (literal "| ")) mdLines) <> blankline
-  else blockToMarkdown opts $ linesToPara lns
+blockToMarkdown' opts (LineBlock lns) = do
+  variant <- asks envVariant
+  case variant of
+    PlainText -> do
+      let emptyToBlank l = if isEmpty l then blankline else l
+      mdLines <- mapM (liftM emptyToBlank . inlineListToMarkdown opts) lns
+      return $ vcat mdLines <> blankline
+    _ ->
+      if isEnabled Ext_line_blocks opts
+      then do
+        mdLines <- mapM (inlineListToMarkdown opts) lns
+        return $ vcat (map (hang 2 (literal "| ")) mdLines) <> blankline
+      else blockToMarkdown opts $ linesToPara lns
 blockToMarkdown' opts b@(RawBlock f str) = do
   variant <- asks envVariant
   let Format fmt = f

test/command/jabberwocky.md

@@ -0,0 +1,22 @@
+```
+% pandoc -t plain -f markdown
+| 'Twas brillig, and the slithy toves
+|       Did gyre and gimble in the wabe:
+| All mimsy were the borogoves,
+|       And the mome raths outgrabe.
+|
+| "Beware the Jabberwock, my son!
+|       The jaws that bite, the claws that catch!
+| Beware the Jubjub bird, and shun
+|       The frumious Bandersnatch!"
+^D
+’Twas brillig, and the slithy toves
+      Did gyre and gimble in the wabe:
+All mimsy were the borogoves,
+      And the mome raths outgrabe.
+
+“Beware the Jabberwock, my son!
+      The jaws that bite, the claws that catch!
+Beware the Jubjub bird, and shun
+      The frumious Bandersnatch!”
+```