LaTeX writer: Don't escape underscore in labels.

Previously they were escaped as ux5f.

Closes #2921.

src/Text/Pandoc/Writers/LaTeX.hs

@@ -310,7 +310,7 @@ toLabel z = go `fmap` stringToLaTeX URLString z
  where go [] = ""
        go (x:xs)
          | (isLetter x || isDigit x) && isAscii x = x:go xs
-         | elem x ("-+=:;." :: String) = x:go xs
+         | elem x ("_-+=:;." :: String) = x:go xs
          | otherwise = "ux" ++ printf "%x" (ord x) ++ go xs
 
 -- | Puts contents into LaTeX command.