[bug] JSONata mutates bindings passed as arguments during evaluate call

[asennikov]

I've noticed by accident that JSONata mutates the variable passed to the evaluate method call, if evaluation fails due to an error associated with that variable, here's the reproduction branch: #589

it("should not mutate bound variable on failure", function() {
    var expr = jsonata("$myVariable()");
    var myVariable = { "foo": "bar" };
    try {
        expr.evaluate(testdata2, { myVariable });
    } catch (e) {
        // ignore the evaluation error
    }
    expect(myVariable).to.deep.equal({ "foo": "bar" });
});