Texinfo writer: Escape special characters in node titles.

This fixes a problem pointed out by Joost Kremers.  Pandoc used
to escape an '@' in a chapter title, but not in the corresponding
node title, leading to invalid texinfo.

src/Text/Pandoc/Writers/Texinfo.hs

@@ -344,7 +344,8 @@ inlineListToTexinfo lst = mapM inlineToTexinfo lst >>= return . hcat
 -- | Convert list of inline elements to Texinfo acceptable for a node name.
 inlineListForNode :: [Inline]  -- ^ Inlines to convert
                   -> State WriterState Doc
-inlineListForNode = return . text . filter (not . disallowedInNode) . stringify
+inlineListForNode = return . text . stringToTexinfo .
+                    filter (not . disallowedInNode) . stringify
 
 -- periods, commas, colons, and parentheses are disallowed in node names
 disallowedInNode :: Char -> Bool