Org reader: allow toggling header args

Org-mode allows to skip the argument of a code block header argument if
it's toggling a value.

This fixes jgm#2269.

src/Text/Pandoc/Readers/Org.hs

@@ -513,7 +513,10 @@ rundocBlockClass :: String
 rundocBlockClass = rundocPrefix ++ "block"
 
 blockOption :: OrgParser (String, String)
-blockOption = try $ (,) <$> orgArgKey <*> orgParamValue
+blockOption = try $ do
+  argKey <- orgArgKey
+  paramValue <- option "yes" orgParamValue
+  return (argKey, paramValue)
 
 inlineBlockOption :: OrgParser (String, String)
 inlineBlockOption = try $ (,) <$> orgArgKey <*> orgInlineParamValue
@@ -525,7 +528,10 @@ orgArgKey = try $
 
 orgParamValue :: OrgParser String
 orgParamValue = try $
-  skipSpaces *> many1 (noneOf "\t\n\r ") <* skipSpaces
+  skipSpaces
+    *> notFollowedBy (char ':' )
+    *> many1 (noneOf "\t\n\r ")
+    <* skipSpaces
 
 orgInlineParamValue :: OrgParser String
 orgInlineParamValue = try $

tests/Tests/Readers/Org.hs

@@ -1094,6 +1094,15 @@ tests =
                    , ": 65" ] =?>
            rawBlock "html" ""
 
+      , "Source block with toggling header arguments" =:
+        unlines [ "#BEGIN_SRC sh :noeval"
+                , "echo $HOME"
+                , "#END_SRC"
+                ] =?>
+        let classes = [ "bash", "rundoc-block" ]
+            params = [ ("rundoc-language", "sh"), ("noeval", "yes") ]
+        in codeBlockWith ("", classes params) "echo $HOME"
+
       , "Example block" =:
            unlines [ "#+begin_example"
                    , "A chosen representation of"