Add a correct check for number with default value

Otherwise on line 116 in Expression.js when n was 0 it was replaced by 1 and then : 

    var solver = new c.SimplexSolver();
    var x = new c.Variable({ name: "x" });
    var y = new c.Variable({ name: "y" });
    solver.addConstraint( new c.Equation(x, 12));
    solver.addConstraint( new c.Equation(y, new c.Expression(x).times(0)) );
    solver.resolve();
    console.log(x.value + " - " + y.value);

Output :   

    "12 - 12"