Text.Pandoc.Lua: respect metatable when getting filters

This change makes it possible to define a catch-all function using lua's
metatable lookup functionality.

    function catch_all(el)
      …
    end

    return {
      setmetatable({}, {__index = function(_) return catch_all end})
    }

A further effect of this change is that the map with filter functions
now only contains functions corresponding to AST element constructors.

1 files changed, 6 insertions, 0 deletions

diff --git a/test/Tests/Lua.hs b/test/Tests/Lua.hs
index 895b93775..06f100048 100644
--- a/test/Tests/Lua.hs
+++ b/test/Tests/Lua.hs
@@ -64,6 +64,12 @@ tests = map (localOption (QuickCheckTests 20))
       "single-to-double-quoted.lua"
       (doc . para . singleQuoted $ str "simple")
       (doc . para . doubleQuoted $ str "simple")
+
+  , testCase "Count inlines via metatable catch-all" $
+    assertFilterConversion "filtering with metatable catch-all failed"
+      "metatable-catch-all.lua"
+      (doc . para $ "four words, three spaces")
+      (doc . para $ str "7")
   ]
 
 assertFilterConversion :: String -> FilePath -> Pandoc -> Pandoc -> Assertion