bug: date objects arent handled

[bricejlin]

Inputs aren't handling date objects correctly. The issue is in flattenObject method https://github.com/skaterdav85/validatorjs/blob/master/src/validator.js#L160

I found that you can add an additional check there current instanceof Date, which should fix it. Let me know if you want me to add that.

[iamdtang]

@LKay, can you take a look since you worked on this? Thanks!

[LKay]

@garygreen Sure, I'll do it along with #102 probably tomorrow as today I'm fully booked.

[iamdtang]

@LKay @bricejlin so is this already taken care of in #105 ?

[LKay]

It seems so. I added test for Date object and it is valid for both node and phantomjs tests. I tested it locally and seems correct too.

The only weird thing about date validation is that some browsers don't treat some formats as valid even though they should be valid by JS specification. I could use moment which seems to handle that fine, but I think this might be an overkill for just simple date validation.