Async Polymorphic relations don't serialize correctly

[aaronbhansen]

If you have a belongsTo that is async and polymorphic, when it tries to serialize the model, it throws an error because it didn't unwrap the promise.

This can be seen here in the console assert. http://emberjs.jsbin.com/zisoba/8/edit

The quick solution locally was for me to unwrap it (https://gist.github.com/aaronbhansen/5a4bb73cf4c16b22b924), but that might be a good longterm fix.

[luketheobscure]

Just ran into this myself. It seems like it should either be fixed, documented, or at least throw an error when you try to declare it as such.

I'm willing to fix this bug, but I would like guidance from one of the project maintainers on how they would like it fixed.

[igorT]

SHould be fixed by #2623