Calling an table with a __call metamethod from C# fails

This fails:

``` csharp
dynamic tab = lua.NewTable("tab");
dynamic mt = lua.NewTable("mt");
mt.__call = new Func<dynamic, object>((t) => "call no args");
tab.SetMetatable(mt);
lua.tab()); //fail
```

```
System.InvalidCastException: Das Objekt des Typs "NLua.LuaTable" kann nicht in Typ "NLua.LuaFunction" umgewandelt werden.
```
