RST reader: don't allow single-dash separator in headerless table.

Closes #4382.

src/Text/Pandoc/Readers/RST.hs

@@ -1269,7 +1269,9 @@ simpleTableHeader headless = try $ do
   rawContent  <- if headless
                     then return ""
                     else simpleTableSep '=' >> anyLine
-  dashes      <- simpleDashedLines '=' <|> simpleDashedLines '-'
+  dashes      <- if headless
+                    then simpleDashedLines '='
+                    else simpleDashedLines '=' <|> simpleDashedLines '-'
   newline
   let lines'   = map snd dashes
   let indices  = scanl (+) 0 lines'

test/command/4382.md

@@ -0,0 +1,10 @@
+```
+% pandoc -f rst -t native
+-
+
+=====
+^D
+[BulletList
+ [[]]
+,HorizontalRule]
+```