Incorrect response for get-unsat-assumptions

Hello, I noticed z3 can produce an incorrect response for `get-unsat-assumptions`. For example:

```
(set-option :produce-unsat-assumptions true)
(check-sat-assuming (false))
(get-unsat-assumptions)
```

The output from z3 is:

```
unsat
()
```

I believe the response to `get-unsat-assumptions` in this case should be `(false)`, since `(check-sat-assuming ())` outputs `sat`.
