Child promise state not set to :pending immediately after #execute when parent has completed

[pelly]

1. Create a parent promise and wait for it to complete
2. Create a child promise chain off the parent (via a call to then, on_success, etc)
3. Call execute on the child promise
4. Immediately check the state of the child promise
5. This state will be :unscheduled.

Expectation: It should be :pending.

[pitr-ch]

Thank you for the fix!