Add a test for external functions with callbacks

This tracks rvirding/luerl#191 which fixes a
decoder bug causing state issues when mixing Elixir and Lua functions.

mix.exs

@@ -55,7 +55,7 @@ defmodule Lua.MixProject do
   # Run "mix help deps" to learn about dependencies.
   defp deps do
     [
-      {:luerl, "~> 1.2"},
+      {:luerl, path: "../../oss/luerl"},
       {:ex_doc, "~> 0.31", only: :dev, runtime: false}
     ]
   end

test/lua_test.exs

@@ -219,6 +219,36 @@ defmodule LuaTest do
       assert lua |> Lua.decode!(table) |> Lua.Table.as_map() == my_table
     end
 
+    test "it can register functions that take callbacks that modify state" do
+      require Logger
+
+      lua = ~LUA"""
+      state = {}
+
+      function assignFoo()
+        state["foo"] = "bar"
+      end
+
+      function assignBar()
+        state["bar"] = "foo"
+      end
+
+      assignBar()
+      run(assignFoo)
+
+      return state
+      """
+
+      assert {[ret], _lua} =
+               Lua.new()
+               |> Lua.set!([:run], fn [callback], lua ->
+                 Lua.call_function!(lua, callback, [])
+               end)
+               |> Lua.eval!(lua)
+
+      assert Lua.Table.as_map(ret) == %{"foo" => "bar", "bar" => "foo"}
+    end
+
     test "it can evaluate chunks" do
       assert %Lua.Chunk{} = chunk = ~LUA[return 2 + 2]c