Fixed if-then-else not activating its propagators.

IfThenElse is bugged because it is not activating the "then" or "else" propagators when the other is not entailed. For example:

```java
public static void main(String[] args) {
    Solver solver = new Solver();
    BoolVar v = VariableFactory.bool("v", solver);
    IntVar i1 = VariableFactory.enumerated("i1", 0, 10, solver);
    IntVar i2 = VariableFactory.enumerated("i2", 0, 10, solver);
    solver.post(LogicalConstraintFactory.ifThenElse(v,
            IntConstraintFactory.arithm(i1, "=", 3),
            IntConstraintFactory.arithm(i2, "=", 3)));
    solver.post(LogicalConstraintFactory.ifThenElse(v,
            IntConstraintFactory.arithm(i2, "=", 0),
            IntConstraintFactory.arithm(i1, "=", 0)));

    solver.set(IntStrategyFactory.firstFail_InDomainMin(new IntVar[]{i1, i2}));

    if (solver.findSolution()) {
        do {
            System.out.println(solver);
        } while (solver.nextSolution());
    }
}
```

Should have exactly two solutions: (i1=3, i2=0) and (i1=0, i2=3). The current implementation has a ton of solutions.