Lua Utils module: improve stringify

Meta value strings (MetaString) and booleans (MetaBool) are now
converted to the literal string and the lowercase boolean name,
respectively. Previously, all values of these types were converted to
the empty string.

src/Text/Pandoc/Lua/Module/Utils.hs

@@ -32,10 +32,11 @@ module Text.Pandoc.Lua.Module.Utils
 
 import Prelude
 import Control.Applicative ((<|>))
+import Data.Char (toLower)
 import Data.Default (def)
 import Foreign.Lua (Peekable, Lua, NumResults)
 import Text.Pandoc.Class (runIO, setUserDataDir)
-import Text.Pandoc.Definition (Pandoc, Meta, MetaValue, Block, Inline)
+import Text.Pandoc.Definition (Pandoc, Meta, MetaValue (..), Block, Inline)
 import Text.Pandoc.Lua.StackInstances ()
 import Text.Pandoc.Lua.Util (addFunction)
 
@@ -111,7 +112,13 @@ stringify el = return $ case el of
   InlineElement i  -> Shared.stringify i
   BlockElement b   -> Shared.stringify b
   MetaElement m    -> Shared.stringify m
-  MetaValueElement m -> Shared.stringify m
+  MetaValueElement m -> stringifyMetaValue m
+
+stringifyMetaValue :: MetaValue -> String
+stringifyMetaValue mv = case mv of
+  MetaBool b   -> map toLower (show b)
+  MetaString s -> s
+  _            -> Shared.stringify mv
 
 data AstElement
   = PandocElement Pandoc