Work around tagsoup bug - not allowing uppercase x in hex entities.

Issue submitted at tagsoup.

src/Text/Pandoc/Parsing.hs

@@ -574,6 +574,7 @@ characterReference = try $ do
   char '&'
   ent <- many1Till nonspaceChar (char ';')
   let ent' = case ent of
+                  '#':'X':xs -> '#':'x':xs  -- workaround tagsoup bug
                   '#':_  -> ent
                   _      -> ent ++ ";"
   case lookupEntity ent' of

src/Text/Pandoc/XML.hs

@@ -107,6 +107,7 @@ fromEntities ('&':xs) =
                              (zs,';':ys) -> (zs,ys)
                              (zs,    ys) -> (zs,ys)
           ent' = case ent of
+                      '#':'X':xs -> '#':'x':xs  -- workaround tagsoup bug
                       '#':_ -> ent
                       _     -> ent ++ ";"