RST reader: fix parsing of headers with trailing space.

This was a regression in pandoc 2.0.

Closes #4280.

src/Text/Pandoc/Readers/RST.hs

@@ -352,7 +352,7 @@ singleHeader' :: PandocMonad m => RSTParser m (Inlines, Char)
 singleHeader' = try $ do
   notFollowedBy' whitespace
   lookAhead $ anyLine >> oneOf underlineChars
-  txt <- trimInlines . mconcat <$> many1 (do {notFollowedBy newline; inline})
+  txt <- trimInlines . mconcat <$> many1 (notFollowedBy blankline >> inline)
   pos <- getPosition
   let len = sourceColumn pos - 1
   blankline

test/command/4280.md

@@ -0,0 +1,7 @@
+```
+% pandoc -f rst -t native
+Driver 
+------
+^D
+[Header 1 ("driver",[],[]) [Str "Driver"]]
+```