Usa metatable para configuração default.

[SOMA-3827][SOMA-4135]

sga/configuration.lua

@@ -5,9 +5,11 @@ local safer = require("safer")
 local schema = require("schema")
 local util = require("sga.util")
 
-local function get(config_data, filename, config)
-   local config = util.table_shallow_copy(config) or {}
-   config.sgas = nil -- avoid multiple inside multiple
+local function get(config_data, filename, config_default)
+   local config = {}
+   config_default.sgas = nil -- avoid multiple inside multiple
+   config_default.__index = function (t,k) return config_default[k] end
+   setmetatable(config, config_default)
    local chunk, err = load(config_data, filename, "t", config)
    if not chunk then
       return nil, "Failed loading config file "..filename..": "..err