[java?] Int vs. Null<Int> overloads

This:

```haxe
class Main {
static function main() {
test(0);
}

@:overload
static function test(value:Int) {}

@:overload
static function test(value:Null) {}
}
```

currently fails with

```
source/Main.hx:7: characters 2-36 : Another overloaded field of same signature was already declared : test
source/Main.hx:10: characters 2-42 : The second field is declared here
```

I'm not sure but I think this might be a regression from when we changed `Null` to be an abstract. There's some code in `overloads.ml` which tries to dance around this situation and still checks for `TType`. The assumption was probably that the only `TType` that would still be there after a `follow_skip_null` would be the old Null-typedef.

@waneck Could you confirm that this kind of overloading was supposed to be allowed in your original implementation?