EitherType in function types

```haxe
class Test {
static function test(f:haxe.extern.EitherType->Void) {
f(123);
f("abc");
}
static function main() {
test(function(v:Int) {
trace(Std.is(v, Int)); //should be always true, but actually print true then false
});
}
}
```

I think the `test()` call shouldn't type-check. i.e. `Int->Void` shouldn't unifies with `haxe.extern.EitherType->Void`.