Soundness bug on QF_S formula (assert (= a (str.replace "A" a "")))

Hi,
For this case, Z3  gives an incorrect answer:
```
[549] % z3-4.8.7 small.smt2
unsat
[550] % cvc4 -q --strings-exp small.smt2
unsat
[551] % z3 small.smt2
sat
[552] % 
[552] % cat small.smt2
(declare-fun a () String)
(assert (= a (str.replace "A" a "")))
(check-sat)
[553] %
```

OS: Ubuntu 18.04
Commit: 8fe3caa
