array().includes() should not allow undefined

[hueniverse]

I am not sure what's the right way of allowing undefined values in an array but the default should not be to allow it. It is rarely the desired or expected outcome.

        it('errors on undefined value', function (done) {

            var schema = Joi.object({
                test: Joi.array().includes(Joi.number())
            });
            var input = { test: [undefined] };

            schema.validate(input, function (err, value) {

                expect(err).to.exist();
                done();
            });
        });

Reported in hapijs/hapi#2138

[Marsup]

@hueniverse Are you ok with the api defined in this commit ?

[hueniverse]

WFM