Update json.lua from pending pull request

rxi/json.lua#51

Author: winniehell

src/resources/pandoc/datadir/_json.lua

@@ -24,6 +24,21 @@
 
 local json = { _version = "0.1.2" }
 
+-- taken from https://www.lua.org/pil/19.3.html
+function pairsByKeys (t, f)
+  local a = {}
+  for n in pairs(t) do table.insert(a, n) end
+  table.sort(a, f)
+  local i = 0      -- iterator variable
+  local iter = function ()   -- iterator function
+    i = i + 1
+    if a[i] == nil then return nil
+    else return a[i], t[a[i]]
+    end
+  end
+  return iter
+end
+
 -------------------------------------------------------------------------------
 -- Encode
 -------------------------------------------------------------------------------
@@ -86,7 +101,7 @@ local function encode_table(val, stack)
 
   else
     -- Treat as an object
-    for k, v in pairs(val) do
+    for k, v in pairsByKeys(val) do
       if type(k) ~= "string" then
         error("invalid table: mixed or invalid key types")
       end
@@ -385,4 +400,4 @@ function json.decode(str)
 end
 
 
-return json
+return json