fix models for Booleans

[iguerNL]

For this example:

(declare-const s Bool)
(declare-const a Bool)
(declare-const b Bool)

(assert
 (not 
  (=
   (= s (or a b))
   (= (not a) (or (not s) b))
   )
  )
 )

(check-sat)

Alt-Ergo returns the following model:

(
  (s s)
  (a s)
  (b b)
)
; File "foo.smt2", line 14, characters 1-12:I don't know (0.0039) (6 steps) (goal g)
unknown

Because Booleans are not interpreted in theories