Self-referential __isa can blow up the call stack when passed through an extension function

This is a fun one:

```Lua
Foo = {}
Foo.__isa = Foo

map.errCheck = function()
    return self
end function

a = new Foo //-- or just Foo.
if a.errCheck isa Foo then print 1
```

`Runtime error: Maximum call stack size exceeded`