Markdown reader: don't try to read contents in self-closing HTML tag.

Previously we had problems parsing raw HTML with self-closing
tags like `<col/>`. The problem was that pandoc would look
for a closing tag to close the markdown contents, but the
closing tag had, in effect, already been parsed by `htmlTag`.

This fixes the issue described in
<https://groups.google.com/d/msgid/pandoc-discuss/297bc662-7841-4423-bcbb-534e99bbba09n%40googlegroups.com>.

src/Text/Pandoc/Readers/Markdown.hs

@@ -1121,6 +1121,7 @@ rawTeXBlock = do
 rawHtmlBlocks :: PandocMonad m => MarkdownParser m (F Blocks)
 rawHtmlBlocks = do
   (TagOpen tagtype _, raw) <- htmlTag isBlockTag
+  let selfClosing = "/>" `T.isSuffixOf` raw
   -- we don't want '<td>    text' to be a code block:
   skipMany spaceChar
   indentlevel <- (blankline >> length <$> many (char ' ')) <|> return 0
@@ -1134,7 +1135,9 @@ rawHtmlBlocks = do
                  gobbleAtMostSpaces indentlevel
                  notFollowedBy' closer
                  block
-  contents <- mconcat <$> many block'
+  contents <- if selfClosing
+                 then return mempty
+                 else mconcat <$> many block'
   result <-
     try
     (do gobbleAtMostSpaces indentlevel