Lua: allow returning a single filter from filter files

It is now possible to return a single filter from a filter file, e.g.

``` lua
-- Switch single- and double quotes
return {
  Quoted = function (q)
    elem.quotetype = elem.quotetype == 'SingleQuote'
      and 'DoubleQuote' or 'SingleQuote'
    return elem
  end
}

The filter must not contain numerical indexes, or it might be treated as
a list of filters.

1 files changed, 0 insertions, 2 deletions

diff --git a/pandoc-lua-engine/test/lua/single-to-double-quoted.lua b/pandoc-lua-engine/test/lua/single-to-double-quoted.lua
index b985b215c..29843df06 100644
--- a/pandoc-lua-engine/test/lua/single-to-double-quoted.lua
+++ b/pandoc-lua-engine/test/lua/single-to-double-quoted.lua
@@ -1,10 +1,8 @@
 return {
-  {
     Quoted = function (elem)
       if elem.quotetype == "SingleQuote" then
         elem.quotetype = "DoubleQuote"
       end
       return elem
     end,
-  }
 }